EDATE
The result is a date which is a number of months away from the start date. Only months are considered; days are not used for calculation.
EDATE(StartDate; Months)
StartDate is a date.
Months is the number of months before (negative) or after (positive) the start date.
When entering dates as part of formulas, slashes or dashes used as date separators are interpreted as arithmetic operators. Therefore, dates entered in this format are not recognized as dates and result in erroneous calculations. To keep dates from being interpreted as parts of formulas use the DATE function, for example, DATE(1954;7;20), or place the date in quotation marks and use the ISO 8601 notation, for example, "1954-07-20". Avoid using locale dependent date formats such as "07/20/54", the calculation may produce errors if the document is loaded under different locale settings.
Unambiguous conversion is possible for ISO 8601 dates and times in their extended formats with separators. If a #VALUE! error occurs, then unselect Generate #VALUE! error in Collabora Office - PreferencesTools - Options - Collabora Office Calc - Formula, button Details... in section "Detailed Calculation Settings", Conversion from text to number list box.
What date is one month prior to 2001-03-31?
=EDATE("2001-03-31";-1) returns the serial number 36950. Formatted as a date, this is 2001-02-28.