Add and subtract time from a DateTime structure in Erlang

Some languages, like Ruby, give you 12 ways to do the same thing. Other languages, like Erlang, make it tough to find 1 way to do something.

Awhile back, I was trying to add a period of time to an existing DateTime value (in {{Y,M,D},{H,m,s}} format), but I couldn't find a function (such as in the Calendar module) that allowed me to manipulate a DateTime value directly.

If you're looking too, the Calendar module can be used to convert a DateTime to seconds, which makes it easier to add seconds, minutes, hours, etc.

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

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

To make life easier, I created a function to add additional time to (or subtract time from) an existing DateTime:

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

This is trivial to write, but then it's trivial for them to just include too. Add it to the things I miss in a modern framework like .NET.