ISO 8601:1988 specifies that dates and times should be written in one of the following formats:
YYYY-MM-DD hh:mm:ss
YYYY-Www-d
YYYY-ddd
YYYY: Year (0000-9999)
MM: Month (01-12)
DD: Day of the month (01-31)
hh: Hour of the day (00-23)
mm: Minutes of the hour (00-59)
ss: Seconds of the minute (00-59)
W: A Literal 'W'
ww: Week of the year, W01 is the week containing January 4 (01-53)
d: Day of the week, Monday is 1, Sunday is 7 (1-7)
ddd: Day of the year, (001-366)
When storing the dates in a machine readable text format all the punctuation marks may be omitted but ISO recommends placing a 'T' between the date and time. For example 20000204T235959.
Unlike any national conventions for specifying dates the ISO standard avoids all ambiguities. If you see the date 2001-02-04 you can be certain it refers to the fourth day of February in the year 2001.