Created: [Apr 6, 2004]

Defines a TRIGGER iCalendar component property.

    4.8.6.3 Trigger

       Property Name: TRIGGER

       Purpose: This property specifies when an alarm will trigger.

       Value Type: The default value type is DURATION. The value type can be
       set to a DATE-TIME value type, in which case the value MUST specify a
       UTC formatted DATE-TIME value.

       Property Parameters: Non-standard, value data type, time zone
       identifier or trigger relationship property parameters can be
       specified on this property. The trigger relationship property
       parameter MUST only be specified when the value type is DURATION.

       Conformance: This property MUST be specified in the "VALARM" calendar
       component.

       Description: Within the "VALARM" calendar component, this property
       defines when the alarm will trigger. The default value type is
       DURATION, specifying a relative time for the trigger of the alarm.
       The default duration is relative to the start of an event or to-do
       that the alarm is associated with. The duration can be explicitly set

       to trigger from either the end or the start of the associated event
       or to-do with the "RELATED" parameter. A value of START will set the
       alarm to trigger off the start of the associated event or to-do. A
       value of END will set the alarm to trigger off the end of the
       associated event or to-do.

       Either a positive or negative duration may be specified for the
       "TRIGGER" property. An alarm with a positive duration is triggered
       after the associated start or end of the event or to-do. An alarm
       with a negative duration is triggered before the associated start or
       end of the event or to-do.

       The "RELATED" property parameter is not valid if the value type of
       the property is set to DATE-TIME (i.e., for an absolute date and time
       alarm trigger). If a value type of DATE-TIME is specified, then the
       property value MUST be specified in the UTC time format. If an
       absolute trigger is specified on an alarm for a recurring event or
       to-do, then the alarm will only trigger for the specified absolute
       date/time, along with any specified repeating instances.

       If the trigger is set relative to START, then the "DTSTART" property
       MUST be present in the associated "VEVENT" or "VTODO" calendar
       component. If an alarm is specified for an event with the trigger set
       relative to the END, then the "DTEND" property or the "DSTART" and
       "DURATION' properties MUST be present in the associated "VEVENT"
       calendar component. If the alarm is specified for a to-do with a
       trigger set relative to the END, then either the "DUE" property or
       the "DSTART" and "DURATION' properties MUST be present in the
       associated "VTODO" calendar component.

       Alarms specified in an event or to-do which is defined in terms of a
       DATE value type will be triggered relative to 00:00:00 UTC on the
       specified date. For example, if "DTSTART:19980205, then the duration
       trigger will be relative to19980205T000000Z.

       Format Definition: The property is defined by the following notation:

         trigger    = "TRIGGER" (trigrel / trigabs)

         trigrel    = *(

                    ; the following are optional,
                    ; but MUST NOT occur more than once

                      (";" "VALUE" "=" "DURATION") /
                      (";" trigrelparam) /

                    ; the following is optional,
                    ; and MAY occur more than once

                      (";" xparam)
                      ) ":"  dur-value

         trigabs    = 1*(

                    ; the following is REQUIRED,
                    ; but MUST NOT occur more than once

                      (";" "VALUE" "=" "DATE-TIME") /

                    ; the following is optional,
                    ; and MAY occur more than once

                      (";" xparam)

                      ) ":" date-time