Note: The following is an excerpt from the official IETF RFC. It is provided here as a convenience and is not authoritative. Refer to the original document as the authoritative reference.
Introduction
Date and time formats cause a lot of confusion and interoperability
problems on the Internet. This document addresses many of the
problems encountered and makes recommendations to improve consistency
and interoperability when representing and using date and time in
Internet protocols.
This document includes an Internet profile of the ISO 8601 [ISO8601]
standard for representation of dates and times using the Gregorian
calendar.
There are many ways in which date and time values might appear in
Internet protocols: this document focuses on just one common usage,
viz. timestamps for Internet protocol events. This limited
consideration has the following consequences:
All dates and times are assumed to be in the “current era”,
somewhere between 0000AD and 9999AD.
All times expressed have a stated relationship (offset) to
Coordinated Universal Time (UTC). (This is distinct from some
usage in scheduling applications where a local time and location
may be known, but the actual relationship to UTC may be dependent
on the unknown or unknowable actions of politicians or
administrators. The UTC time corresponding to 17:00 on 23rd March
2005 in New York may depend on administrative decisions about
daylight savings time. This specification steers well clear of
such considerations.)
Timestamps can express times that occurred before the introduction
of UTC. Such timestamps are expressed relative to universal time,
using the best available practice at the stated time.
Date and time expressions indicate an instant in time.
Description of time periods, or intervals, is not covered here.