homepage

For what parts of ISO 8601 to accept, there's a standard: RFC3339, "Date and Time on the Internet: Timestamps". See section 5.6:
 date-fullyear = 4DIGIT
 date-month = 2DIGIT ; 01-12
 date-mday = 2DIGIT ; 01-28, 01-29, 01-30, 01-31 based on
 ; month/year
 time-hour = 2DIGIT ; 00-23
 time-minute = 2DIGIT ; 00-59
 time-second = 2DIGIT ; 00-58, 00-59, 00-60 based on leap second
 ; rules
 time-secfrac = "." 1*DIGIT
 time-numoffset = ("+" / "-") time-hour ":" time-minute
 time-offset = "Z" / time-numoffset
 partial-time = time-hour ":" time-minute ":" time-second
 [time-secfrac]
 full-date = date-fullyear "-" date-month "-" date-mday
 full-time = partial-time time-offset
 date-time = full-date "T" full-time
 NOTE: Per [ABNF] and ISO8601, the "T" and "Z" characters in this
 syntax may alternatively be lower case "t" or "z" respectively.
 ISO 8601 defines date and time separated by "T".
 Applications using this syntax may choose, for the sake of
 readability, to specify a full-date and full-time separated by
 (say) a space character.
That's straightforward, and can be expressed as a regular expression.