Date.prototype.setDate()

The setDate() method of Date instances changes the day of the month for this date according to local time.

If you specify a number outside the expected range, the date information in the Date object is updated accordingly. For example, if the Date object holds June 1st, a dateValue of 40 changes the date to July 10th, while a dateValue of 0 changes the date to the last day of the previous month, May 31st.

Because setDate() operates on the local time, crossing a Daylight Saving Time (DST) boundary may result in a different elapsed time than expected. For example, if setting the date crosses a spring-forward transition (losing an hour), the difference in timestamps between the new and old date is one hour less than the nominal day difference multiplied by 24 hours. Conversely, crossing a fall-back transition (gaining an hour) result in an extra hour. If you need to adjust the date by a fixed amount of time, consider using setUTCDate() or setTime().

Syntax example: setDate(dateValue)