Date manipulator: Perform operations and format dates

This class is meant to perform operations on dates.

It can add or subtract days, months or years to a given date. It can also:

- Change from date numbers to text.
- Get the current date.
- Get the day, month and year values of a date separately.
- Get the name of the day of the week or name of the month
- Convert timestamp to a formatted date
- Change the date format