seconds – convert human–readable date (and time) to seconds since epoch

seconds date ...

Seconds prints the number of seconds since 1 Jan 1970 corresponding to one or more human–readable dates. Each date must be one argument; it will usually be necessary to enclose it in quotes.

Seconds accepts a somewhat wider range of input than just output from date(1). The main requirement is that the date must be fully specified, with a day of month, month and year in any order. The month must be an English name (or abbreviation), not a number, and the year must contain 4 digits. Unambiguous time–zone names are understood (i.e., not IST) or time zones may be written as ±hhmm. Case is ignored.

Print the names of all files under . modified since the start of 23 May 2011.
du –ta | awk '$1 >= '^`{seconds '23 may 2011'}^' {print $2}'

date(1), du(1), mtime(1), ctime(2)

All–numeric dates, popular in the USA, are simply ambiguous, more so if the year is truncated to 2 digits.