Isn't this what Date::Calc is for? It also seems that if you wanted roll your own you could convert back to epoch (with POSIX), add seconds, and the do localtime on the result.

UPDATE</b: Upon further inspection the premise of my roll-your-own is similar to what you are doing, I approve ;-). However I believe the suggestion meets your requirement of simplifying the process, YMMV.

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perl -pe "s/\b;([st])/'\1/mg"