c_date: Perform basic arithmetic calculations with dates

This is a simple class that performs basic arithmetic calculations with dates.

It can add or subtract an amount of days to a given date.

It can also calculate the difference of days between two dates, returning fractions of days as result. Use the round() or floor() to eliminate fractions.

It uses the strtotime() function to convert the dates into 32 bit integer timestamp values to perform the calculations. Therefore, the class is limited to dates that start on 1970-01-01.