dateTimeNewFromHttpString
Parses @date_string and tries to extract a date from it.
This recognizes all of the "HTTP-date" formats from RFC 2616, RFC 2822 dates, and reasonable approximations thereof. (Eg, it is lenient about whitespace, leading "0"s, etc.)
Return
a new #GDateTime, or null if @date_string could not be parsed.
Parameters
dateString
The date as a string