What date falls a given number of days away, and which ISO week it is.
Add the days to the start date, crossing month boundaries and allowing for month lengths. Thirty days from 15 January is 14 February, not the 15th, because January has thirty-one days. That one-day shift is the usual mental-arithmetic error.
A month keeps the day number, while thirty days counts a fixed span. One month from 31 January is 28 February, since the nearest valid day is the month's last, whereas thirty days from the same date is 2 March. In contracts that gap matters.
Under Latvian civil law, a period whose last day falls on a Saturday, Sunday or public holiday ends on the next working day. The calculator gives the calendar date, so check afterwards whether it is a working day and move it forward if not.
Enter a negative number of days or choose subtraction, and the calculation walks back through the calendar the same way. It is useful for working out when to start: if an event is on 20 May and preparation takes forty-five days, you begin on 5 April.