Date Util: Format and manipulate dates in several ways

This class can be used to format and manipulate dates in several ways. It can:

- Return a date in a given format
- Validate a date string
- Retrieve the current date in a given time zone
- Compute the difference of days between two dates
- Add a given number of days, months or years to a date
- Determine the day of the week of a given date
- Compute the Unix timestamp for a given date