[FEATURE] Time computations

[tommens]

Describe the new functionality you would like to see
It would be nice to be able to compute different time intervals. For instance:

How many hours have elapsed since 18:52?
How long was the interval between may 5, 2020 19:30:32 and march 2, 2020 18:31:19?
Date format is up to the choice of the student, but the ISO format (YYYY-MM-DD) is recommended.
Both the 24-hour and 12-hour time formats need to be taken into account. They can be mixed in requests: e.g. the calculator should be able to answer "03:54 PM - 11:54 = 4 hours".

The application should be able to provide the answer in a unit requested by its user, with the remainder being expressed either fractionally or through sub-units: 2020-05-05 19:30:32 - 2020-03-02 18:31:19 can be expressed as either "64 days, 0 hours, 59 minutes and 13 seconds" or "64.04 days" or "92219 minutes" or "5533153 seconds", ...

Timezones should be taken into account: "05:27 PM CET+0 - 05:27 PM CET+12 = 12 hours".

Subtraction and addition (what day will it be in 173 days?) make sense, multiplication and division do not. Therefore, those operations should not be possible with time computations.

If the user does not specify a start date or time, the current date/time is used. So, a unary request such as "- 18:52" or "+ 00:01" should give the user the time it was 18 hours and 52 minutes ago or the time it'll be in one minute. In this case, the INFIX notation can be considered equivalent to the PREFIX notation.