I'll try to explain the problem I met in a better way (sorry for my english :/).
I live in Europe (Italy), and for that reason, my clock is set 2 hours more than the GMT (1 hour is for the Dayloght saving time).
I had some problems because the rss aggregator my blog was subscribed to, aggregated my posts as if they where been written 2 hours before the real date. But the blog homepage reported the correct time, so I went reading in the code, beginning from wp-rss2.php. There, I found that the "GMT offset" (+0000, +0200) part of the date reported in the feed, was static. Once I changed the date string, using the 'r' parameter of date() (that actually give the same output, but with the gmt offset, working directly on the date settings of the server), and everything went fine.
I've not check if the problem still in 1.5.1, because I patched the two files before trying it... :/