Unfortunately Date::Calc does not install cleanly for me. But I have the same problem with both Perl's built-in time functionality (specifically POSIX::strftime()) and the DateTime module. Both claim that time stamp 391263207 corresponds to 'Sonntag, 26.05.2013 - 14:13:27', using time zones 'CET-1CDT', 'Europe/Berlin', and 'Europe/Busingen', and locale 'de_DE'. I calculate the GMT date for this time stamp as 'Sonntag, 26.05.2013 - 12:13:27'.

So this means Perl thinks Summer time (+2 offset from GMT) is in effect at time stamp 391263207, while your SMS log thinks it was not. I do not live in Germany, so I do not know for sure which is correct. But the end of May seems to me awfully late in the year for Winter time. Or awfully early. Maybe you should step back from the programming problem and try to figure out whether Summer or Winter time was in effect at that time. If it's Summer, then Perl is right.