The international standard to represent dates and times.
To make the long story short: The recommended way to represent date
is YYYY-MM-DD (as in 2001-03-16) and time HH:mm (in 24 hour format, as
in 00:27). To combine them: 2001-03-15T22:27Z (date, "T", time, "Z" to
say that UTC is used.)
Local time is represented as is (00:27), and international time in
UTC format with Z appended (23:27Z, in this case 2 timezones away
because I happen to be in Finland). Seconds and even minutes can be
left out if so required.
Periods of time are represented with START/END (slash in middle);
Thus, 2001-03-16/2001-03-23 represents week from today-of-writing, and
2001-03-16T08/15Z tells when I'm wrecking some cars. =)
(Based on Jukka Korpela's excellent summary "ISO 8601, the date
and time representation standard"...)