QUOTE(Rob Itagaki @ Sep 23 2011, 19:59)

What rad said. It means Greenwich Mean Time. A universal time zone.
13:00 is 1:00 PM on a 24 hour clock. Most countries outside of the US and armed forces use this.
Well, I live in central US (kansas) and have no friends outside the US, so I've never heard of that before, good to know, Wikipedia here I come.