Created: [Apr 6, 2004]

Defines a DURATION iCalendar component property.

3.3.6.  Duration

Value Name:  DURATION

Purpose:  This value type is used to identify properties that contain
a duration of time.

Format Definition:  This value type is defined by the following
notation:

dur-value  = (["+"] / "-") "P" (dur-date / dur-time / dur-week)

dur-date   = dur-day [dur-time]
dur-time   = "T" (dur-hour / dur-minute / dur-second)
dur-week   = 1*DIGIT "W"
dur-hour   = 1*DIGIT "H" [dur-minute]
dur-minute = 1*DIGIT "M" [dur-second]
dur-second = 1*DIGIT "S"
dur-day    = 1*DIGIT "D"

Description:  If the property permits, multiple "duration" values are
specified by a COMMA-separated list of values.  The format is
based on the [ISO.8601.2004] complete representation basic format
with designators for the duration of time.  The format can
represent nominal durations (weeks and days) and accurate
durations (hours, minutes, and seconds).  Note that unlike
[ISO.8601.2004], this value type doesn't support the "Y" and "M"
designators to specify durations in terms of years and months.

The duration of a week or a day depends on its position in the
calendar.  In the case of discontinuities in the time scale, such
as the change from standard time to daylight time and back, the
computation of the exact duration requires the subtraction or
addition of the change of duration of the discontinuity.  Leap
seconds MUST NOT be considered when computing an exact duration.
When computing an exact duration, the greatest order time
components MUST be added first, that is, the number of days MUST
be added first, followed by the number of hours, number of
minutes, and number of seconds.

Negative durations are typically used to schedule an alarm to
trigger before an associated time (see Section 3.8.6.3).

No additional content value encoding (i.e., BACKSLASH character
encoding, see Section 3.3.11) are defined for this value type.

Example:  A duration of 15 days, 5 hours, and 20 seconds would be:

P15DT5H0M20S

A duration of 7 weeks would be:

P7W