Hello,
I have released Tox 0.1.0, a little library for working with DateTime, NaiveDateTime, Date, and Time. It also adds Tox-Period and Tox-Interval. My motivation was to have some date-time calculation functions that work with any time zone database and different calendars. So you can use Tox with TimeZoneInfo, Tz, and Tzdata.
For the tests and examples, I use the Calendar.ISO and the calendars Cldr.Calendar.Coptic, Cldr.Calendar.Ethiopic, and Cldr.Calendar.Persian implemented by @kip.

Some examples:

add:

iex> datetime = DateTime.from_naive!(~N[2020-07-19 12:00:00], "Europe/Berlin")
#DateTime<2020-07-19 12:00:00+02:00 CEST Europe/Berlin>
iex> Tox.DateTime.add(datetime, year: 1, month: 1, day: 1, hour: 1, minute: 1)
#DateTime<2021-08-20 13:01:00+02:00 CEST Europe/Berlin>
iex> Tox.DateTime.add(datetime, year: -1, month: 1, day: 1, hour: 1, minute: 1)
#DateTime<2019-08-20 13:01:00+02:00 CEST Europe/Berlin>
iex> Tox.DateTime.add(datetime, week: 1)    
#DateTime<2020-07-26 12:00:00+02:00 CEST Europe/Berlin>

iex> datetime = DateTime.from_naive!(~N[2020-07-19 12:00:00], "Europe/Berlin") 
...> |> DateTime.convert!(Cldr.Calendar.Coptic)
#DateTime<1736-11-12 12:00:00+02:00 CEST Europe/Berlin Cldr.Calendar.Coptic>
iex> Tox.DateTime.add(datetime, month: 1)
#DateTime<1736-12-12 12:00:00+02:00 CEST Europe/Berlin Cldr.Calendar.Coptic>
iex> Tox.DateTime.add(datetime, month: 2)
#DateTime<1736-13-05 12:00:00+02:00 CEST Europe/Berlin Cldr.Calendar.Coptic>

iex> DateTime.from_naive(~N[2020-03-29 02:30:00], "Europe/Berlin")
{:gap, #DateTime<2020-03-29 01:59:59.999999+01:00 CET Europe/Berlin>,
 #DateTime<2020-03-29 03:00:00+02:00 CEST Europe/Berlin>}

iex> datetime = DateTime.from_naive!(~N[2020-03-29 01:30:00], "Europe/Berlin")
#DateTime<2020-03-29 01:30:00+01:00 CET Europe/Berlin>
iex> Tox.DateTime.add(datetime, hour: 1)
#DateTime<2020-03-29 03:30:00+02:00 CEST Europe/Berlin>

beginning_of_, end_of_:

iex> ~N[2020-07-12 11:12:13]
...> |> DateTime.from_naive!("America/Winnipeg")
...> |> Tox.DateTime.beginning_of_year()
#DateTime<2020-01-01 00:00:00-06:00 CST America/Winnipeg>

iex> ~N[2020-07-12 11:12:13]
...> |> DateTime.from_naive!("Europe/Berlin")
...> |> Tox.DateTime.end_of_year()
#DateTime<2020-12-31 23:59:59.999999+01:00 CET Europe/Berlin>

# In 1994 the year ended one day earlier in the time zone Pacific/Kirimati.
iex> ~N[1994-07-12 11:12:13]
...> |> DateTime.from_naive!("Pacific/Kiritimati")
...> |> Tox.DateTime.end_of_year()
#DateTime<1994-12-30 23:59:59.999999-10:00 -10 Pacific/Kiritimati>

iex> datetime = DateTime.from_naive!(~N[2020-07-15 12:00:00], "Europe/Berlin")
#DateTime<2020-07-15 12:00:00+02:00 CEST Europe/Berlin>
iex> Tox.DateTime.end_of_week(datetime)                                 
#DateTime<2020-07-19 23:59:59.999999+02:00 CEST Europe/Berlin>
iex> Tox.DateTime.end_of_day(datetime)                                  
#DateTime<2020-07-15 23:59:59.999999+02:00 CEST Europe/Berlin>
iex> Tox.DateTime.beginning_of_week(datetime)
#DateTime<2020-07-13 00:00:00+02:00 CEST Europe/Berlin>
iex> Tox.DateTime.beginning_of_day(datetime)
#DateTime<2020-07-15 00:00:00+02:00 CEST Europe/Berlin>

iex> datetime = DateTime.from_naive!(~N[2020-07-19 12:00:00], "Europe/Berlin")
#DateTime<2020-07-19 12:00:00+02:00 CEST Europe/Berlin>
iex> datetime = DateTime.convert!(datetime, Cldr.Calendar.Coptic)       
#DateTime<1736-11-12 12:00:00+02:00 CEST Europe/Berlin Cldr.Calendar.Coptic>
iex> Tox.DateTime.end_of_year(datetime)                                 
#DateTime<1736-13-05 23:59:59.999999+02:00 CEST Europe/Berlin Cldr.Calendar.Coptic>

Tox.Period, Tox.Interval:

iex> now = DateTime.from_naive!(~N[2020-07-12 21:33:43], "America/New_York")
iex> period = Tox.Period.new!(month: 1)
#Tox.Period<P1M>
iex> interval = Tox.Interval.new!(Tox.DateTime.beginning_of_month(now), period)
#Tox.Interval<[2020-07-01T00:00:00-04:00/P1M[>
iex> Tox.Interval.contains?(interval, now)
true
iex> Tox.Interval.since_start(interval, now, :millisecond)
{:ok, 1028023000}
iex> Tox.Interval.until_ending(interval, now, :millisecond)
{:ok, 1650377000}
iex> Tox.Interval.until_ending(interval, Tox.DateTime.add(now, month: 1), :millisecond)
:error