Again, as an alternative to POSIX, you can get a lot of nicely parsed offset functionality with Date::Manip.

$ perl -MDate::Manip -e 'print ParseDate("14 days ago")."\n"'
2003061110:22:53
$ perl -MDate::Manip -e 'print UnixDate("14 days ago","%Y%m%d")."\n"'
20030611
$ perl -MDate::Manip -e 'print localtime(UnixDate("14 days ago","%s"))
+."\n"'        
Wed Jun 11 10:23:04 2003
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-xdg