What is the difference between calendar months and fixed days?

A calendar month respects the target month's actual length, while a fixed number of days always moves the same distance regardless of which months it happens to cross.

When do the two month-end rules actually produce different results?

Only when the base date is itself the last day of its month. Any other starting day produces the same result under either rule.

Date Calculator

Add or subtract calendar time from a date. Choose calendar units (which respect month length) or fixed days (which never do), and pick how a target day that doesn't exist in the resulting month should be handled.

Want to compare all three month-end rules side by side? See Month-End Roll Calculator.

Want to learn more?

Worked example

January 31, 2027, plus 1 calendar month constrains to February 28, since February has no 31st that year. The same date in 2028, a leap year, plus 1 month instead gives February 29, since that month has 29 days in a leap year. Adding 30 fixed days instead of a calendar month, regardless of month length, to that same January 31, 2027, lands on March 2, a different date entirely from either calendar-month result.

Common mistakes

Assuming calendar months and fixed days always land on the same date. They only coincide by chance, and diverge as soon as a crossed month is not exactly 30 days long.
Not choosing a month-end rule deliberately. Constrain and preserve end-of-month only disagree when the base date is itself the last day of its month, but that case is common enough to matter.

Constrain versus preserve end-of-month

Both rules only differ when the base date is itself the last day of its month. Constrain always lands on the resulting month's last valid day for an out-of-range target. Preserve end-of-month goes further, keeping the result on the last day of its month even across a chain of additions, so a monthly end-of-month schedule stays anchored to month-end throughout, not just on the first step.

Limitations

This is calendar arithmetic only. It does not determine a contractual or billing due date, so the actual governing document should always be checked for its own specific convention.

Frequently asked questions

What is the difference between calendar months and fixed days?: A calendar month respects the target month's actual length, while a fixed number of days always moves the same distance regardless of which months it happens to cross.
When do the two month-end rules actually produce different results?: Only when the base date is itself the last day of its month. Any other starting day produces the same result under either rule.