EOMONTH: month boundaries without guessing the calendar

Month lengths change; report boundaries should not depend on a manually updated list of 28, 30 and 31-day months. EOMONTH gives you a dependable anchor.

Month lengths do not need a manually maintained 28/30/31-day table. An offset of 1 returns the end of the following month, not a fixed number of days after the start date. Adding one to a month-end gives the first day of the next month.

Put =DATE(2026,9,12) in A2 and format outputs as dates.

Generate month-end and next-month boundaries

Formula syntax

=EOMONTH(start_date, months)

What goes into it

A quick example first

=EOMONTH(DATE(2026,9,12),0)

=EOMONTH(A2,1)

Before you copy this into a real sheet

For transactions that include times, use >= the first day and < the first day of the next month. A <= month-end test includes midnight but can exclude the remainder of that last day. EOMONTH produces a numeric date serial; seeing a number usually means the cell needs date formatting. Confirm the input is a real date with ISNUMBER and use DATE rather than an ambiguous quoted date string in portable examples.

Useful habits

• Use 0 for the end of the current month.
• Add 1 to EOMONTH(A2,-1) to get the first day of A2’s month.
• Keep start_date as a true date value, not ambiguous text.

Where people get caught out

• Supplying text that looks like a date but is not parsed consistently.
• Forgetting that the result may display as a serial number if the cell is not date-formatted.

Other functions worth a look

A detail worth remembering

For timestamped transactions, use an inclusive start date and an exclusive first day of the next month. This avoids excluding the final afternoon of a reporting month.