def parse_time_string(timestring):
result = _parse_gmt_time(timestring)<EOL>if result:<EOL><INDENT>return calendar.timegm(result)<EOL><DEDENT>return None<EOL>
Return the number of seconds since the epoch, for a date/time string.

    Returns None for invalid input

    The following time type strings are supported:

    Sun, 06 Nov 1994 08:49:37 GMT  ; RFC 822, updated by RFC 1123
    Sunday, 06-Nov-94 08:49:37 GMT ; RFC 850, obsoleted by RFC 1036
    Sun Nov  6 08:49:37 1994       ; ANSI C's asctime() format