A) To calculate the number of days between two dates:
- Use top row date pickers. Click left button on the first date  or fill out the date field  manually;
- Click left button on the second date or fill out the date field manually. Then you see result.

B) To calculate next date:
- Use bottom row date picker.Click left button on the date  or fill out the date field manually;
- Input in the middle field  number days. In the right fields you see result. Possible set negative number of days also.

In both events, you may include (exclude) some weekdays and holidays.

