Time Specification Reference

vCard

RFC 2426 (vCard) - 4.Formal Grammar - Page37:

4.  Formal Grammar
[...]
   ;*******************************************
   ; vCard Type Value Definition
   ;*******************************************
[...]
   date-value   = <A single date value as defined in [MIME-DIR]>

   time-value   = <A single time value as defined in [MIME-DIR]>

   date-time-value = <A single date-time value as defined in [MIME-DIR]
[...]

(MIME-DIR: RFC 2425 - MIME Content-Type for Directory Information)

RFC 2425 (MIME-DIR) - 5.8.4. Pre-defined Value Types - Page12:

[...]
   date = date-fullyear ["-"] date-month ["-"] date-mday

   date-fullyear = 4 DIGIT
   date-month = 2 DIGIT     ;01-12

   date-mday = 2 DIGIT      ;01-28, 01-29, 01-30, 01-31
                            ;based on month/year

   time = time-hour [":"] time-minute [":"] time-second [time-secfrac]
           [time-zone]

   time-hour = 2 DIGIT      ;00-23

   time-minute = 2 DIGIT    ;00-59

   time-second = 2 DIGIT    ;00-60 (leap second)

   time-secfrac = "," 1*DIGIT

   time-zone = "Z" / time-numzone

   time-numzome = sign time-hour [":"] time-minute

vCalendar1.0

http://www.imc.org/pdi/vcal-10.txt

[...]
Date and Time
The date and time values for all vCalendar properties are formatted as a string consistent with the ISO 8601 representation for combinations of dates and times. Either the basic or extended format is allowed. The use of UTC, rather than local time, should be used when ever possible in order to avoid time zone ambiguities. The format for the complete, basic representation of a date and time value is written in the following sequence of characters:
<year><month><day>T<hour><minute<second><type designator>
For example, 8:30 AM on April 15, 1996 local time would be written as:
19960415T083000
And the same time in UTC based time would be written as:
19960415T083000Z
Where a value needs to specify a sequence of date and time values, then the property value is a string made up of a list of date and time values, separated by the field separator. For example:
19960101T090000Z; 19960201T090000Z; 19960301T090000Z; 19960401T090000Z; ...
[...]

iCalendar

RFC 2445 (iCalendar) - 4.3 Property Value Data Types:

4.3.4 Date - Page34

[...]
     date               = date-value

     date-value         = date-fullyear date-month date-mday
     date-fullyear      = 4DIGIT
     date-month         = 2DIGIT        ;01-12
     date-mday          = 2DIGIT        ;01-28, 01-29, 01-30, 01-31
                                        ;based on month/year
[...]
   Example: The following represents July 14, 1997:

     19970714
[...]

4.3.5 Date-Time - Page35

[...]
   Formal Definition: The value type is defined by the following
   notation:

     date-time  = date "T" time ;As specified in the date and time
                                ;value definitions
[...]
   For example, the following represents Janurary 18, 1998, at 11 PM:

     DTSTART:19980118T230000
[...]

4.3.12 Time - Page47

[...]
   Formal Definition: The data type is defined by the following
   notation:

     time               = time-hour time-minute time-second [time-utc]

     time-hour          = 2DIGIT        ;00-23
     time-minute        = 2DIGIT        ;00-59
     time-second        = 2DIGIT        ;00-60
     ;The "60" value is used to account for "leap" seconds.

     time-utc   = "Z"
[...]
   FORM #1 LOCAL TIME

   The local time form is simply a time value that does not contain the
   UTC designator nor does it reference a time zone. For example, 11:00
   PM:

     230000
[...]
   FORM #2: UTC TIME

   UTC time, or absolute time, is identified by a LATIN CAPITAL LETTER Z
   suffix character (US-ASCII decimal 90), the UTC designator, appended
   to the time value. For example, the following represents 07:00 AM
   UTC:

     070000Z
[...]