Network Working Group J. Noerenberg
ietf-draft-calsch-mod-01.txt Qualcomm, Inc
Category: INTERNET DRAFT Expires: Jan 1998
Internet Calendar Model Specification
Status of this Memo
This document is an Internet-Draft. Internet-Drafts are working
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Distribution of this document is unlimited.
Abstract
Internet Calendaring and Scheduling protocols require the definition
of objects to encapsulate an event to be scheduled, a calendar on
which the event will be stored, and means for exchanging these objects
across the Internet between calendars on behalf of people for whom the
calendars are meaningful. This document gives an abstract model of the
objects and the protocols necessary to accomplish this kind of
information exchange. It will establish the context in which other
Calendaring and Scheduling RFCs can be interpreted. Included are
brief introductions to the other components of Internet calendar
protocols and definitions of nomenclature common to all documents
defining these protocols. Reading this document will enable
implementors and users of Internet Calendaring and Scheduling
protocols to understand the goals and constraints chosen for related
protocols.
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Table of Contents
1. Document framework 3
1.1 Model Specification 3
1.2 iCalendar: Core Object Specification 3
1.3 iTIP: Transport Independent Interoperability Protocol 3
1.4 CAP: Calendar Access Protocol Specification 4
2. Abstract Model 5
3. Principal Model Components 6
3.1 Calendar User 6
3.2 Calendar 7
3.3 Calendar User Agent (CUA) 8
3.4 Calendar Service 8
3.5 Calendar Domain 9
3.6 Calendar Access Protocol (CAP) 9
3.7 Transport Independent Interoperability Protocol (iTIP) 9
4. Calendar Object Transport 10
4.1 Direct Access 11
4.2 Calendar Service Mediation 11
4.3 Interdomain Exchange 12
5. Security considerations 12
6. References 13
7. Acknowledgments 14
8. Author's address 14
9. Appendix -- Calendar protocol nomenclature 15
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Introduction
The Internet Calendar Model specification provides a framework for the
set of protocols that define Calendaring and scheduling for the
Internet. The protocols specify the contents of calendars, and how
the objects stored on calendars are represented during transmission
across the Internet. These protocols also define the interaction
between a calendar user agent and a calendar store, as well as the
actions performed by calendar transfer agents that facilitate
communication between calendar stores. These terms will be defined
more precisely in the section on nomenclature below. The protocols
are the:
"Core Object Specification" [iCalendar]
"iCalendar Interoperability Protocol" [iTIP]
"Calendar Access Protocol" [CAP]
1. Document framework Calendar and Scheduling Protocols are contained
in a series of related documents. This section describes the
relationship between the documents. Section 2 presents the abstract
model for Internet Calendaring and Scheduling.
Following sections amplify the principal concepts defined in the
abstract model, provide a schematic representation of information flow
in Internet Calendaring and Scheduling, and supply other, useful
background information.
1.1 Model Specification This document - see abstract and introduction
above.
1.2 iCalendar: Core Object Specification The Core Object Specification
is the authoritative definition of all properties that may be used in
the Internet Calendar and Scheduling Protocols as well as the rules
for encoding and representing the objects that are constructed from
those properties. iCalendar also specifies the method to be used to
define new attributes.
1.3 iTIP: Transport Independent Interoperability Protocol This
document specifies how calendaring systems use iCalendar objects to
interoperate with other calendar systems. It does so in a general way
so as to allow multiple methods of communication between systems.
Binding of iTIP to a real-time protocol This document specifies a
session-layer iTIP protocol. Multiple bindings are possible. This WG
will specify and foster implementation of at least one binding.
Binding of iTIP to E-mail This document specifies an iTIP protocol
over Internet e-mail using MIME. Internet e-mail protocols are given
by RFCs 821, 822, 2045-2049 [RFC-821] [RFC-822] [RFC- 2045] [RFC-2046]
[RFC-2047]. [RFC-2048] [RFC-2049]. See the references for details for
constructing Internet e-mail messages.
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1.4 CAP: Calendar Access Protocol Specification This document
specifies how a Calendar User Agent (CUA) will interact with a
Calendar Service using iCalendar objects.
A graphical representation of the relationship between the documents
is shown below:
------------------
| Model Document |
------------------
|
|
|
------------------
| iCalendar |
------------------
|
|
|
-----------------------------------
| |
------------------ ------------------
| iTIP | | CAP |
------------------ ------------------
|
-----------------
| |
---------- -----------
| session | | email |
| iTIP | | iTIP |
---------- -----------
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2. Abstract Model
A CALENDAR is a collection of logically related OBJECTs.
OBJECTs include EVENTs, TODOs, JOURNALs, ALARMs, TIMEZONEs and
FREEBUSYs.
Each OBJECT is described using a set of OBJECT ATTRIBUTES such as
date&time, attendees, resources and statuses.
The complete set of OBJECT ATTRIBUTES and their representation is
defined in [iCalendar].
A specific CALENDAR has a unique CALENDAR IDENTIFIER (CI).
A CALENDAR USER (CU) views and modifies a CALENDAR using a CALENDAR
USER AGENT (CUA). Via a CUA a CU can modify a CALENDAR by adding or
modifying OBJECTs stored in the CALENDAR. A CUA uses the services of
a CALENDAR SERVICE (CS) via the CALENDAR ACCESS PROTOCOL (CAP) to
publish changes to a CALENDAR. A CS also delivers messages containing
OBJECTs from other CALENDAR USERs via CAP, or via the iCalendar
Transport-Independent Interoperability Protocol, iTIP.
A CS stores a set of CALENDARs which are accessible according to
ACCESS RULES maintained by the CS. CAP enables the CUA and CS to
request access to CALENDARs according to the ACCESS RULES. CAP also
enables a CUA and CS to modify the current ACCESS RULES for particular
CALENDAR USERs and particular CALENDARs.
iTIP supports the exchange of OBJECTs without the use of ACCESS RULES.
It enables the exchange of objects between CSs, as well as between
CUAs and CSs. A set of CSs may cooperate in a CALENDAR DOMAIN. A
CALENDAR DOMAIN appears to be a single CS through iTIP, from the point
of view of another CS.
A minimal INTERNET CALENDAR SYSTEM comprises a CUA, a CS and the
services of CAP and iTIP, all of which which may, or may not, be
integrated together in a given implementation.
NOTE: It is not required that interacting CUA, CS, CAP and iTIP
entities be matched implementations, though it is required that all
implementations must comply with the specified CAP and iTIP.
A CS is responsible for locating the appropriate CALENDAR DOMAIN for
CIs specified in OBJECTs to be transmitted between CALENDAR DOMAINS. A
CUA is also required to locate the appropriate CALENDAR DOMAIN in
order to use iTIP. When OBJECTs are transmitted, they are
encapsulated in a CALENDAR PROTOCOL DATA UNIT (CPDU).
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CPDUs are MIME encoded objects that specify a requested action and/or
response and carry associated data. Thus, the format of all
information exchanged among CALENDAR SYSTEM ENTITIES is defined in
terms of MIME CONTENT-TYPES with associated PARAMETER VALUES and MIME
BODY PART CONTENT. CONTENT is defined in terms of iCalendar.
The complete set of CPDUs and the corresponding finite state machine
for unauthenticated exchange make up iTIP. iTIP is defined in
[iTIP1], [iTIP2], and [iTIP3]. iTIP is capable of using a variety of
transport mechanisms including INTERNET MAIL ([RFC821], [RFC822]),
HTTP, [RFC2091], as well as session-layer protocols derived directly
from iTIP.
ACCESS RULES and the cooresponding finite state machine for
authenticated exchange make up CAP. CAP is defined in [CAP].
3. Principal Model Components
There are several principal components in a Calendaring and Scheduling
system. Their relationship can be seen in Figure 2 below. This
section identifies some of the salient features of the components. The
syntax and semantics for creating and transmitting these objects are
completely specified in [iCalendar], [CAP], and [iTIP].
3.1 Calendar User
A calendar user interprets objects on a calendar, creates them, and
exchanges them with other calendar users. A calendar user may be a
person (Ken Caminiti), a group of people (the the San Diego Padres
baseball club), or a resource (Qualcomm Stadium). From the point of
view of other calendar users, groups and resources appear as a single
Calendar user, regardless of their composition in the physical world.
Calendar users that are resources generally contain properties that
identify them as inanimate objects -- anything from a fruit bowl to
rubber bats to settle disputes during a meeting.
A calendar user owns his own calendar, and can manipulate objects
stored there via a CUA. Access control attributes condition access to
calendars and their components and properties.
A calendar user can also manipulate the contents of other calendar
users' calendars by sending messages containing calendar objects to
them. For example, The San Diego Padres sends calendar events for the
1997 season to Ken Caminiti, so he knows when to show up at the
ballpark. The Padres sends calendar events for games to be played at
home to the Qualcomm Stadium calendar so the concessionaires can order
hot dogs.
A calendar user can also organize and own events. When a calendar
user creates an event object, that user becomes the organizer and the
owner. The organizer can delegate ownership and the role of organizer
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to others. Only organizers and owners may alter any attribute of an
event object. Calendar users assigned other Attendees roles may not
change event attributes.
3.2 Calendar
3.2.1 Collection of objects
Calendar users own calendars. This is a one to many relationship. A
single calendar user may have multiple calendars. However, each
calendar must have a unique identifier. A calendar is an information
store containing information about events,to-dos, alarms, journals and
free time, the objects stored in it. Also stored in a calendar are
properties that are global to all of the objects in the calendar. An
example of a global property is the GEO property that identifies the
physical location where the calendar user can be found. Global
properties such as this establish the context used to interpret the
objects stored in the calendar. The principal structural features of
a calendar are described below in section 3.2.3. When objects or
properties of a calendar are exchanged between actors in a calendaring
and scheduling network (Calendar User Agents and Calendar Services),
they are expressed in the form defined in [iCalendar]. Figure 1 below
is a schematic representation of a calendar.
3.2.2 Properties
Properties are attributes of an object or a calendar. They consist of
a name and a value. Properties are strongly typed. Some properties
are multivalued. A property may have parameters that distinguish
between related properties. Some properties may occur multiple times
in the same object instance, and may be gathered into a logical group.
Some properties may be unique to a particular calendar or object.
3.2.3 Objects
Objects are collections of property values. A particular set of
values for the properties of a object define a particular object.
Some objects may contain certain other objects. The set of objects in
a calendar are identified below.
3.2.3.1 Events
Event objects are defined for specific starting date-time, have a
duration on a calendar, and a description. Other properties of an
event may specify a location or other attributes that define the
event, such as resources that are part of the event. Events may also
contain an Alarm object.
3.2.3.2 To-do
While like an event, a To-do doesn't reserve a specific block of time
on a calendar. A To- do component must have a starting date-time that
identifies its first appearance on the calendar. It must also have a
date-time that specifies when the To-do expires. A To-do must have a
description. It may also contain an alarm object.
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3.2.3.3 Alarms
An alarm object may occur in an Event or To-do. It contains a
date-time. When present, and the date-time is passed, it will cause a
CUA or CS to notify the user the date-time has passed.
3.2.3.4 Journal
A journal object is a textual item that is associated with a
particular date. As its name suggests, its purpose is to record
information meaningful about the date, but not necessarily tied to
other calendar objects on a calendar.
3.2.3.5 Timezone
Timezone objects encapsulate rules for calculating local time from
UTC. Including this object in an event object enables a receiver to
calculate the universal time value for time values expressed in the
sender's local time. This object is especially useful for expressing
recurring events whose instances span a change in the time reference
such as the transition between Standard time and Daylight Savings
time. Time values expressed in local time are always interpreted in
the receiver's local time. The sender must provide another context
using UTC values and Timezone objects if this is not the
interpretation intended by the sender.
calendar
|
-----------------------------------------------------------
| | | | | |
to-do event journal freebusy timezone property...
|
--------------
| |
property... alarm
|
property...
Figure 1: Calendar Object Model
3.3 Calendar User Agent (CUA)
A CUA mediates the interactions between a calendar user and his
calendar. It represents the information stored in the calendar to the
user, and enables the user to manipulate it. This is a particular
instance of the interactive process used by a calendar user.
3.4 Calendar Service
A Calendar Service (CS) stores a collection of calendars and interacts
with the Calendar User Agent (CUA) via the Calendar Access Protocol
(CAP).
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3.5 Calendar Domain
A collection of calendars that can be grouped together constitutes a
Calendar Domain. The relation used to bound the group is arbitrary.
Frequently membership in an organization will be used to define the
domain, but it could be a shared Internet address domain, as well. A
Calendar Domain provides a contiguous address space for all the
calendars, CTAs and CUAs contained in the domain. It must be possible
for any Calendar User (via the facilities of a CUA and/or CTA) to
determine whether they are members of a particular domain, or if other
Calendar Users are members. CTAs and CUAs can take advantage of
domain information when routing event messages.
3.6 Calendar Access Protocol (CAP)
When calendar users need to manipulate calendars that are not stored
on the same computer where the CUA executes, the CUA will use the
Calendar Access Protocol to exchange objects with the Calendar Service
(CS). CAP specifies the beginning and ending of the session between
the CUA and the CS. Using CAP, the CUA will mediate authentication of
the user to the service. CAP requires calendar objects and calendar
properties to be expressed in the on-the-wire data format defined in
[CalObjSpec]. A CUA must not be required to maintain a connection to a
CS via CAP in order to display a Calendar for a Calendar User or
accept commands from a user to change a Calendar's content. By using
CAP a CUA need not have specific information on how a particular CS
stores a Calendar and vice versa. This enables specification and
exchange of objects and properties independently of Calendar storage
models adopted by particular CUAs or CSs.
3.7 Transport Independent Interoperability Protocol (iTIP)
CSs in a domain or across domains exchange objects and properties
using iTIP. Like CAP, iTIP uses iCalendar formats to represent objects
and properties. iTIP defines the beginning and ending of the exchange
session, as well the users for whom the messages are intended. iTIP
permits unauthenticated delivery of objects and properties to a CS. A
CS may accept or reject delivery without interaction with a user. But
a CS may require further confirmation of receipt of a object or
property before it is acted upon by the CS.
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4. Calendar Object Transport
There are several transport modes in this architecture. The figures
below illustrate the different modes that are allowed. Three modes
are required to handled the different kinds of calendar exchanges
across the Internet, person to person, group interactions local to a
particular network, and exchanges across networks.
------------ ------------
| CUA | rcvr| ----- ----|rcvr| CUA |
| -----| \ / |---- |
| | \ / | |
| | X | |
| | / \ | |
| -----| / \ |---- |
| | sndr| ----- ----|sndr| |
------------ \ / ------------
iTIP
Figure 2: Direct Access
------------ ------------
| CUA | | CUA |
| | | |
| | | |
------------ ------------
\ /
\ ------- CAP ------- /
\ /
\ -------------- /
\ | CS | /
| |
| |
--------------
Figure 3: Calendar Service Mediation
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----------------- ------------------
| | | |
| Calendar Domain | | Calendar Domain |
| | | |
| | | |
| ------- | | |
| | CUA | | | |
| ------- | | |
| | | | |
| | -- CAP | | |
| | | | |
| ------------ | |--------- |
| | CS | rcvr|---------- -------|rcvr| | |
| | -----| | \ / |---- | |
| | | | \ / | Gateway | |
| | | | X | | |
| | | | / \ | | |
| | -----| | / \ |---- | |
| | | sndr| --------- ------|sndr| | |
| ------------ | \ / |--------- |
| | iTIP | |
| | | |
----------------- ------------------
Figure 4: Interdomain Exchange
4.1 Direct Access
For direct access, two calendar user agents (CUA) exchange calendar
components by using iTIP. Each CUA provides an iTIP sender and
receiver. As is generally the case, the methods used by the CUA to
store calendar data locally are not relevant to the transport model.
For this mode, calendar users must be uniquely identifiable across the
Internet, and access to CUAs must conform with privacy and security
considerations. Because the transport itself is not authenticated, it
is crucial the objects themselves be capable of carrying
authentication information.
4.2 Calendar Service Mediation
Using Calendar Service Mediation a CUA interoperates with a Calendar
Service (CS) using CAP to exchange calendar components. The CS takes
responsibility for mediating receipt and delivery of components with
other collaborating CUAs. The principal difference between this mode
and Interdomain Exchange is that CSs do not need to use iTIP to
facilitate exchange of components. A consequence of this mode is that
CUAs and CSs need not use addresses that are unique across the
Internet. However, consistency with other modes makes a consistent
address model an obvious simplification for implementors. Moreover,
because CAP access provides authentication, objects exchanged through
a CAP channel need not carry authenication information.
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4.3 Interdomain Exchange
With Interdomain Exchange a Calendar Service (CS) supporting a group
of users in one domain can exchange calendar components with a
different calendar domain. Domains may or may not be within the same
Internet network domain. Like Direct Access, iTIP is the vehicle
which permits component exchange. In the figure, one domain is
illustrated with a Calendar Service providing iTIP service. The 2nd
domain in this figure has a distinct iTIP sender and receiver. In
order to provide end-to-end privacy components must be wrapped in a
cryptographically secure wrapper to insure only the intended
corespondents can interpret the components. This wrapper is not
required unless privacy must be assured. In order to provide backward
compatibility with existing calendar and scheduling systems, a privacy
wrapper cannot be a required aspect of the component exchange.
5. Security considerations
The Core Object Specification must provides the means to classify the
intended sensitivity level of an event, to-do or journal calendar
component (i.e., PUBLIC, PRIVATE, or CONFIDENTIAL). It must also
provide a means to wrap all components in an exchange in a
cryptographically secure envelope so that only the intended
correspondents can decode the message.
The Calendar Access Protocol must provide a description of the
elements of the calendaring system security model and the protocol
elements for creating and manipulating the access control to a
calendar. This protocol must also describe the authentication
procedure between a CUA and CS.
So that iCalendar objects may be securely transmitted across the
Internet and may be verified by recipients, iCalendar must describe
how objects will be covered and authenticated.
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6. References
[RFC-821] Postel, J., "Simple Mail Transfer Protocol", STD 10, RFC
821, USC/Information Sciences Institute, August 1982.
[RFC-822] Crocker, D., "Standard for the Format of ARPA Internet Text
Messages", STD 11, RFC 822, UDEL, August 1982.
[RFC-2045] Borenstein, N. & Freed, N., "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message Bodies", RFC
2045, Nov 1996
[RFC-2046] Borenstein, N. & Freed, N., "Multipurpose Internet Mail
Extensions MIME) Part Two: Media Types", RFC 2046, Nov 1996
[RFC-2047] Borenstein, N. & Freed, N., "MIME (Multipurpose Internet
Mail Extensions) Part Three: Message Header Extensions for Non-ASCII
Text", RFC 2047, Nov 1996
[RFC-2048] Borenstein, N. & Freed, N., "Multipurpose Internet Mail
Extensions (MIME) Part Four: Registration Procedures", RFC 2048, Nov
1996
[RFC-2049] Borenstein, N. & Freed, N., "Multipurpose Internet Mail
Extensions (MIME) Part Five: Conformance Criteria and Examples", RFC
2049, Nov 1996
[RFC-2068] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Berners-Lee, T., "Hypertext Transfer Protocol -- HTTP/1.1", RFC 2068,
Jan 1997
[iCalendar] Dawson, F. & Stenerson, D., "Internet Calendaring and
Scheduling Core Object Specification" ietf-calsch-ical-01.txt, March,
1997
[iTIP1] Silverberg, S., Mansour, S., Dawson, F., & Hopson, R.,
"iCalendar Transport-Independent Interoperability Protocol (iTIP) Part
One: Scheduling Events and BusyTime",
draft-ietf-calsch-itip-part1-00.txt, Jul 1997
[iTIP2] Silverberg, S., Mansour, S., Dawson, F., & Hopson, R.,
"iCalendar Transport-Independent Interoperability Protocol (iTIP) Part
Two: Scheduling To-Dos", draft-ietf-calsch-itip-part2-00.txt, Jul 1997
[iTIP3] Silverberg, S., Mansour, S., Dawson, F., & Hopson, R.,
"iCalendar Transport-Independent Interoperability Protocol (iTIP) Part
Three: Scheduling Journal Entries",
draft-ietf-calsch-itip-part3-00.txt, Jul 1997
[CAP] "Calendar Access Protocol"
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7. Acknowledgments
The author is extremely grateful for the assistance, patience and
tolerance of the members of the CalSch working group. Their ideas and
suggestions are crucial to making this a useful document. In
particular, the author would like to thank
Anik Ganguly
Derik Stenerson
Frank Dawson
Gilles Fortin
Einar Stefferud
Their comments and ideas were particularly important in the
formulation of this draft. I would also like to thank Qualcomm,
Incorporated for allowing the time necessary to bring this effort to
fruition.
8. Author's address
For more information, please contact the author via Internet Mail.
John W. Noerenberg, II
Qualcomm, Incorporated
6455 Lusk Blvd
San Diego, CA 92131
USA
email: jwn2@qualcomm.com
ph: +1 619 658 3510
The "Internet Calendar Model Specification" is the work of the
Internet
Engineering Task Force Working Group on Calendaring and Scheduling.
The chairman of that group, Anik Ganguly, may be reached at:
Anik Ganguly
Campbell Services, Inc.
21700 Northwestern Highway
10th Floor
Southfield, MI 48075
email: anik@ontime.com
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9. Appendix -- Calendar protocol nomenclature
Calendaring and Scheduling uses a rich lexicon of terms that are
specific to the problems of scheduling events and reconciling
conflicting requests for time and resources. This document will
identify the major components of these systems, and show component
relationships. However, for the sake of completeness and to serve as
an introduction to the protocols in general, a more extensive list of
terms, and brief definitions are included here. Essential parts of the
system model have expanded definitions in this document where the
components of the model are introduced.
9.1 Calendaring lexicon
Alarm, Reminder
An asynchronous mechanism providing feedback for a pending or past
event or to-do.
Busy Time
A period of time that is not available for scheduling.
Calendar
A particular collection of calendar objects.
Calendar Object
The objects that can be found in a calendar. The objects are
events, to-dos, journals, free/busies, time zones and alarms.
Objects also include properties and may include other objects.
A calendar object is identified by unique delimiters.
Calendar Date
A day identified by its position within the calendar year.
Calendar Scale
A particular type of calendar year, for example, Gregorian, Buddhist
Era, Japanese Emperor Era, Chinese Lunar, Islamic, and Jewish
Calendars.
Calendar Service
A Calendar Service (CS) stores a collection of calendars and interacts
with the Calendar User Agent (CUA) via the Calendar Access Protocol
(CAP).
Calendar User Agent (CUA)
A CUA mediates the interactions between a calendar user and his
calendar. It represents the information stored in the calendar to the
user, and enables the user to manipulate it. This is a particular
instance of the interactive process used by a calendar user.
Coordinate Universal Time (UTC)
The time scale maintained by the Bureau International de l'Heure
(International Time Bureau). UTC is often incorrectly referred to as
GMT.
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Daylight Saving Time (DST)
An adjustment to local time to accommodate annual changes in the
number of daylight hours. DST is also known as Advanced Time, Summer
Time, or Legal Time. Daylight Saving Time adjustments in the Southern
Hemisphere are opposite to those in the Northern Hemisphere.
Event
A calendar object that defines a scheduled activity, minimally
specified by a start and end calendar date and time of day and a
description.
Free Time
A period of time available for scheduling on a calendar.
FreeBusy
FreeBusy objects describe blocks of allocated and unallocated time
on a calendar. They do not contain a description why the time is
allocated.
Gregorian Calendar
A calendar scale in general use beginning in 1582. The Gregorian
Calendar scale is based on a solar calendar consisting of common years
made up of 365 days and leap years made up of 366 days; both divided
into 12 sequential months.
Journal
A calendar object that defines a collection of information intended
for human presentation and is minimally specified by a calendar date
and one or more descriptions.
Local Time
The clock time in public use in a locale. Local time is often
referenced by the customary name for the time zone in which it is
located. The relationship between local time and UTC is based on the
offset(s) that are in use for a particular time zone. In general, the
formula is as follows:
local time = UTC + (offset)
Period
A duration of time, specified as either a defined length of time or by
its beginning and end points.
Properties
Attributes that apply to calendar objects or calendars. A calendar
object is a named set of properties. Properties can also be used
to produce variants to suit a particular purpose.
Recurrence Rule
A notation used to represent repeating occurrences, or the exceptions
to such a repetition of an event or a to-do. The recurrence rule can
also be used in the specification of a time zone description.
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Repeating Event or To-do
An event or to-do that repeats for one or more additional occurrences.
The recurrence may be defined with discrete dates and times and/or
with a recurrence rule.
Standard Time
Introduced by Sir Sanford Fleming and others around 1870, standard
time is a scheme for dividing the world into zones where the same time
would be kept. The original proposal was to divide the world into 24
zones, each zone having a width of 15 degrees of longitude. The center
zone was originally the meridian passing through Greenwich, England,
called Greenwich Mean Time (GMT). The time in the zones was
decremented by one hour per zone going westwards and was incremented
by one hour per zone going eastwards from GMT. Changes have been made
to the original proposal to accommodate political boundaries. In
addition, some countries and regions specify 30 or 45 minute offsets,
rather than the full 60 minute offset. Standard time is also known as
Winter Time in some regions.
GMT and UTC are generally equivalent. However, by international
agreement, the GMT term is discouraged in favor of the term UTC for
all general time keeping.
Time Zone
A geographic region to which a specified offset from UTC applies.
While offsets can frequently be calculated by knowing the longitudinal
distance from Greenwich, England, local conventions frequently alter
the calculation, complicating the determination of local time. Local
convention may also assign a label to identify the time zone. There
is no world-wide standard for labels.
To-do
A calendar object that defines an action item and is minimally
specified by an effective calendar date and time of day, a due
calendar date and time of day, a priority and a description.
Noerenberg Expires Jan 1998 [Page 17]
john noerenberg
jwn2@qualcomm.com
pager: jwn2@pager.qualcomm.com
