Calculates the date after and before a specific date and number of days.

The date after (before) specified days is calculated from the start date and the number of days.

When the start date and days are specified, and "Calculate the date" button is clicked, the date after a specified day is displayed.

The date before a specified day from the date is calculated by putting the check in "Before the day".

Moreover, the start date is calculated as a day by putting the check in "Include he start date".