Date tools: Manage date values

This class can be used to manage date values.

It can perform several types of operations with dates:

- Check whether a date is correctly formatted
- Convert a date between formats
- Calculate the date that will be in a given number of days or months
- Check whether a given year is a leap year
- Retrieve the number of day in a given month and year
- Calculate the difference between two given dates in years, months and days