DAYS360 in Excel: Compare US and European 30-Day-Month Rules

• Introduced: first introduction version is not established by the official history reviewed; the current product list is not a launch record.
• Availability: official help lists Excel for Microsoft 365, Excel for Microsoft 365 for Mac, Excel 2024, Excel 2024 for Mac, Excel 2021, Excel 2021 for Mac, Excel 2019, Excel 2016.
• Checked: October 14, 2026; official DAYS360 reference. Documentation and independently modeled examples; no native Excel execution or broader platform certification.

=DAYS360(start_date, end_date, [method])

The two dates are required. Method FALSE or omitted selects the documented U.S. NASD method; TRUE selects the European method. Use actual numeric dates such as DATE results and display the answer as Number.

Choosing the correct convention is an accounting or contract requirement outside the formula. These are arithmetic demonstrations, not advice to apply a particular basis to a real agreement. Make the chosen method visible in the workbook.

=DAYS360(DATE(2026,1,15),DATE(2026,2,15),FALSE)
=DAYS360(DATE(2026,1,15),DATE(2026,2,15),TRUE)

Both expected results are 30. The corresponding actual calendar difference is 31 days. There is no contradiction: the two calculations measure different day-count conventions.

For January 15 through March 15, both methods have expected count 60, while the actual non-leap-2026 calendar interval has 59 days. A convention-based count can be either smaller or larger than elapsed calendar days.

Microsoft describes the U.S. method as adjusting a last-day starting date to day 30. Its ending-date adjustment depends on whether the starting day is before day 30: an applicable ending month-end becomes the first of the next month or day 30 of the same month.

The European method changes a starting or ending day 31 to day 30. The worked contrasts below use 31-day month ends to make the two branches explicit. The broad U.S. last-day wording is not treated as a complete native certification of every February or shorter-month edge.

After the relevant adjustments, a useful independent arithmetic model is 360*(end_year-start_year)+30*(end_month-start_month)+end_day-start_day. It models the documented convention; it is not the Excel calculation engine.

=DAYS360(DATE(2026,1,29),DATE(2026,1,31),FALSE)
=DAYS360(DATE(2026,1,29),DATE(2026,1,31),TRUE)

Expected results are 2 and 1. Under the U.S. branch, the January 31 endpoint becomes February 1 because the start day is before 30: 30+1-29=2. Under the European rule it becomes January 30: 30-29=1.

Actual calendar days are 2. That happens to match the U.S. result for this pair, but it does not make DAYS360 a general replacement for DAYS.

=DAYS360(DATE(2026,1,30),DATE(2026,1,31),FALSE)
=DAYS360(DATE(2026,1,30),DATE(2026,1,31),TRUE)

Both expected results are 0, because the adjusted endpoints are both January 30. Actual elapsed calendar time is one day. An output of zero is therefore not proof that the two input dates are identical.

For January 31 through March 31, both expected convention counts are 60, with adjusted days 30 and 30. The actual 2026 calendar interval is 59 days.

=DAYS360(DATE(2026,1,1),DATE(2026,12,31))
=DAYS360(DATE(2026,1,1),DATE(2026,12,31),TRUE)

Expected results are 360 and 359. The omitted method uses the U.S. branch, taking the ending December 31 to January 1 of the next year for this start day. The European branch takes it to December 30.

The actual elapsed difference is 364 calendar days. Dividing the two convention counts by 360 yields 1 and approximately 0.997222222. Those fractions demonstrate the chosen basis; they do not determine which basis a real document requires.

Microsoft states that a start after the end returns a negative count. Reversing the ordinary February 15/January 15 pair gives expected −30 for either method. This simple control avoids assuming that every adjusted month-end pair has identical forward/reverse symmetry.

For a report with start dates in A and end dates in B, keep separate result columns for explicit FALSE, explicit TRUE and the actual B2-A2 difference. Preserve the method and original inputs when reconciling values; do not replace one count merely because it differs from another convention.

The dedicated source does not establish a complete error-code, text-coercion or all-February-boundary matrix here. Those cases remain held for separately verified native tests. In particular, no claim is made that its concise month-end prose covers every historical implementation detail.

DAYS uses end first, while DAYS360 uses start first. DATEDIF measures complete years/months or days under a different contract. Similar-looking date formulas are not interchangeable merely because the ordinary monthly example gives a plausible number.

• Microsoft DAYS360 reference.
• Editorial testing standards.

Expected results are independent calendar/arithmetic models or explicitly documented rules, not captured native output. Examples use English function names and comma separators. Input dates and schedules are fictional.