Date = {{2018,8,14},{13,10,25}}.
DateInSeconds = calendar:datetime_to_gregorian_seconds(Date).  % 63701471425
NewDateInSeconds = DateInSeconds + 10.                         % 63701471435
calendar:gregorian_seconds_to_datetime(NewDateInSeconds).      % {{2018,8,14},{13,10,35}}

Date = {{2018,8,14},{13,10,25}}.
DateInSeconds = calendar:datetime_to_gregorian_seconds(Date).  % 63701471425
NewDateInSeconds = DateInSeconds + (10 * 60 * 60).             % 63701507425 (10 hours)
calendar:gregorian_seconds_to_datetime(NewDateInSeconds).      % {{2018,8,14},{23,10,25}}

-type datetime() :: {{non_neg_integer(), pos_integer(), pos_integer()},
                     {non_neg_integer(), non_neg_integer(), non_neg_integer()}}.
-type timespan() :: {integer(), integer(), integer()}.

-spec add_time_to_datetime(datetime(), timespan()) -> datetime().
add_time_to_datetime(DateTime, {Hour, Min, Sec}) ->
    DateInSeconds = calendar:datetime_to_gregorian_seconds(Date),
    NewDateInSeconds = DateInSeconds + (Hour * 60 * 60) + (Min * 60) + Sec,
    calendar:gregorian_seconds_to_datetime(NewDateInSeconds).