Timezones and Calendar Events
InstantCal uses timezones to display calendar event information according to the preferences of viewers or publishers.
A viewer or publisher may request InstantCal to display calendar event information for a particular timezone. If no preference is indicated by either a viewer or a publisher, then calendar event information is displayed as if the events are in the 'local' timezone (the timezone of the system used to view the events).
A description of timezones, calendar event date and date-time information, and date-time processing are given below.
Timezones
Timezones are defined as geographic regions where time-of-day differs from Greenwich Mean Time (GMT), also known as Coordinated Universal Time (UTC).
Additionally, the time-of-day differences may vary during the calendar year if a timezone includes periods of Standard Time and Daylight Savings Time.
Timezones are abbreviated by a timezone identifier. A set of worldwide timezones and timezone identifiers are defined (see Wikipedia ZoneInfo).
Date and Date-time Information
Calendar events are always associated with one or more date or date-time values.
Calendar events may have:
  • start and end dates (or date-times)
  • a start date (or date-time) and duration
  • only a start date (or date-time)
Date values do not include a time-of-day and so are not subject to adjustment to any particular timezone. That is, 'Saturday' is always 'Saturday', regardless of any event timezone definitions or timezone preferences.
Date-time values do include a time-of-day and may be adjusted to a particular timezone, depending upon whether they are of a floating or a non-floating type.
A floating date-time value has the same value even if the timezone in which it is defined differs from a timezone preference or local timezone where it is displayed (that is, a floating date-time of 11am in New York is also interpreted as 11am in Los Angeles).
A non-floating date-time, however, may be adjusted from the timezone in which it is defined to a timezone preference or local timezone where it is displayed
Further, non-floating date-time values are of two types:
  • utc
  • local time with a timezone reference
A utc date-time is defined for GMT (Greenwich Mean Time). A local time with a timezone reference date-time is defined for a particular timezone, and may be in Standard Time or Daylight Savings Time.
Date-time Processing
InstantCal processes (non-floating) date-time values by adjusting the original values set by the calendar creator to the timezone preference set by the viewer. If the viewer does not set a timezone preference, the adjustment is made to the timezone preference set by the publisher. If the publisher does not set a timezone preference, the adjustment is made to the local timezone.
Summary
Calendar creators define calendar events with dates, floating date-time values (always the same time in any timezone), or non-floating date-time values (which are timezone-dependent).
Calendar viewers and publishers control how (non-floating) date-time information in calendar events is displayed.
Publishers set a timezone preference, or enable viewers to set a timezone preference. If neither viewers nor publishers indicate a timezone preference, non-floating date-time values are displayed in the local timezone of the calendar viewing system.
 
The InstantCal API contains Timezone configuration details.
 
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