Clase DateTimeManager: Add or subtract a period to a date

This is a simple class that can be used to add or subtract a period to a date.

It takes as parameter a date with time in the ISO format as it is used also by MySQL. It can take an empty date string to specify the current date and time.

The class can add or subtract from the specified date and time a value in years, months, day, hours, minutes or seconds.

The resulting date is returned in the ISO format.

The code and comments are in Spanish.

In Spanish:
Esta clase permite moverse en el calendario hacia adelante y atras, se le pasa una fecha, la cantidad de unidades a moverse y la unidad (dia, mes, aņo, hora, minuto o segundo) y nos retorna la fecha resultado en formato MySQL.