HTTP format of date/time

HTTP applications have historically allowed three different formats for the representation of date/time stamps:

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

The first format is preferred as an Internet standard and represents a fixed-length subset of that defined by RFC 1123 (an update to RFC 822). The second format is in common use, but is based on the obsolete RFC 850 date format and lacks a four-digit year. HTTP/1.1 clients and servers that parse the date value MUST accept all three formats (for compatibility with HTTP/1.0), though they MUST only generate the RFC 1123 format for representing HTTP-date values in header fields.

All HTTP date/time stamps MUST be represented in Greenwich Mean Time (GMT), without exception. For the purposes of HTTP, GMT is exactly equal to UTC (Coordinated Universal Time). This is indicated in the first two formats by the inclusion of "GMT" as the three-letter abbreviation for time zone, and MUST be assumed when reading the asctime format. HTTP-date is case sensitive and MUST NOT include additional LWS beyond that specifically included as SP in the grammar.^[1]

Source:
[1] tools.ietf.org/html/rfc2616