You can't just add tuples in erlang. You could add the individual integers inside the tuples but to manipulate time, you should use the calendar
module (docs):
1> Added = calendar:datetime_to_gregorian_seconds(calendar:local_time()) + 90.
63529954043
2> calendar:gregorian_seconds_to_datetime(Added).
{{2013,3,8},{9,27,23}}