DATE returns a date serial number — the same underlying number system that all dates in Excel and Google Sheets use, and the same format accepted by
DATEDIF and the calculate age formulas that depend on valid date inputs. Format the result cell as a date to display it correctly; without date formatting it shows as a number like 45366. The three arguments accept formula results as well as literal numbers, so DATE can be combined with YEAR, MONTH, and DAY to deconstruct an existing date and reconstruct it with modifications. DATE(YEAR(A2),MONTH(A2)+1,1) gives the first day of the following month regardless of how many days the current month has. DATE(YEAR(A2),MONTH(A2),0) gives the last day of the current month by using day 0 of the next month. Month arithmetic with DATE is reliable across month-end boundaries — adding 3 months to October correctly produces January of the next year with no manual year-rollover logic needed. To calculate differences between dates, use DATEDIF or simple subtraction.