You could use the str2time() / time2str() functions instead (which are exported by default, btw).

my $date = "2008-10-29";

my $http_date = time2str( str2time($date) );   # Tue, 28 Oct 2008 23:0
+0:00 GMT
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You can also explicitly specify the time zone with str2time(), e.g.

str2time("2008-10-29",   "GMT")   # --> Wed, 29 Oct 2008 00:00:00 GMT
str2time("2008-10-29", "-0800")   # --> Wed, 29 Oct 2008 08:00:00 GMT
str2time("2008-10-29", "+0100")   # --> Tue, 28 Oct 2008 23:00:00 GMT
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(As to your off-by-one day problem, I'm not really sure I understand what you mean, so I can't comment on that...)