Extract from RFC5545:

3.3.5.  Date-Time

Value Name:  DATE-TIME

Purpose:  This value type is used to identify values that specify a
precise calendar date and time of day.

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

date-time  = date "T" time ;As specified in the DATE and TIME
;value definitions

Description:  If the property permits, multiple "DATE-TIME" values
are specified as a COMMA-separated list of values.  No additional
content value encoding (i.e., BACKSLASH character encoding, see
Section 3.3.11) is defined for this value type.

The "DATE-TIME" value type is used to identify values that contain
a precise calendar date and time of day.  The format is based on
the [ISO.8601.2004] complete representation, basic format for a
calendar date and time of day.  The text format is a concatenation
of the "date", followed by the LATIN CAPITAL LETTER T character,
the time designator, followed by the "time" format.

The "DATE-TIME" value type expresses time values in three forms:

The form of date and time with UTC offset MUST NOT be used.  For
example, the following is not valid for a DATE-TIME value:

19980119T230000-0800       ;Invalid time format

FORM #1: DATE WITH LOCAL TIME

The date with local time form is simply a DATE-TIME value that
does not contain the UTC designator nor does it reference a time
zone.  For example, the following represents January 18, 1998, at
11 PM:

19980118T230000

DATE-TIME values of this type are said to be "floating" and are
not bound to any time zone in particular.  They are used to
represent the same hour, minute, and second value regardless of
which time zone is currently being observed.  For example, an
event can be defined that indicates that an individual will be
busy from 11:00 AM to 1:00 PM every day, no matter which time zone
the person is in.  In these cases, a local time can be specified.
The recipient of an iCalendar object with a property value
consisting of a local time, without any relative time zone
information, SHOULD interpret the value as being fixed to whatever
time zone the "ATTENDEE" is in at any given moment.  This means
that two "Attendees", in different time zones, receiving the same
event definition as a floating time, may be participating in the
event at different actual times.  Floating time SHOULD only be
used where that is the reasonable behavior.

In most cases, a fixed time is desired.  To properly communicate a
fixed time in a property value, either UTC time or local time with
time zone reference MUST be specified.

The use of local time in a DATE-TIME value without the "TZID"
property parameter is to be interpreted as floating time,
regardless of the existence of "VTIMEZONE" calendar components in
the iCalendar object.

FORM #2: DATE WITH UTC TIME

The date with UTC time, or absolute time, is identified by a LATIN
CAPITAL LETTER Z suffix character, the UTC designator, appended to
the time value.  For example, the following represents January 19,
1998, at 0700 UTC:

19980119T070000Z

The "TZID" property parameter MUST NOT be applied to DATE-TIME
properties whose time values are specified in UTC.

FORM #3: DATE WITH LOCAL TIME AND TIME ZONE REFERENCE

The date and local time with reference to time zone information is
identified by the use the "TZID" property parameter to reference
the appropriate time zone definition.  "TZID" is discussed in
detail in Section 3.2.19.  For example, the following represents
2:00 A.M. in New York on January 19, 1998:

TZID=America/New_York:19980119T020000

If, based on the definition of the referenced time zone, the local
time described occurs more than once (when changing from daylight
to standard time), the DATE-TIME value refers to the first
occurrence of the referenced time.  Thus, TZID=America/
New_York:20071104T013000 indicates November 4, 2007 at 1:30 A.M.
EDT (UTC-04:00).  If the local time described does not occur (when
changing from standard to daylight time), the DATE-TIME value is
interpreted using the UTC offset before the gap in local times.
Thus, TZID=America/New_York:20070311T023000 indicates March 11,
2007 at 3:30 A.M. EDT (UTC-04:00), one hour after 1:30 A.M. EST
(UTC-05:00).

A time value MUST only specify the second 60 when specifying a
positive leap second.  For example:

19970630T235960Z

Implementations that do not support leap seconds SHOULD interpret
the second 60 as equivalent to the second 59.

Example:  The following represents July 14, 1997, at 1:30 PM in New
York City in each of the three time formats, using the "DTSTART"
property.

DTSTART:19970714T133000                   ; Local time
DTSTART:19970714T173000Z                  ; UTC time
DTSTART;TZID=America/New_York:19970714T133000
; Local time and time
; zone reference