Last-Modified is defined as "Last-Modified" ":" HTTP-date (rfc2616 section 14.29)
HTTP-date is defined in rfc2616, section 3.3.1:
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.
The ANSI C asctime() format you're using does not include a time zone in its date string as a HTTP-date.