Last Day of Month Calculator

How it is calculated

You enter a year (1 to 9999) and a month number (1 to 12), both whole numbers. The tool builds the first day of that month and moves to the last day of the same month using the Gregorian calendar. The length of each month comes from the fixed table of 31, 30 or 28 days, with February getting 29 when year mod 4 = 0 unless year mod 100 = 0 and year mod 400 ≠ 0. It returns the date as year-month-day, the number of days in the month, and the weekday of that last day in the page language. A month outside 1 to 12, a decimal such as 2.5 or a year of 0 gives no result. Years before the calendar's adoption are counted by the same Gregorian rule, which is a simple extension of it.

Common mistakes

• Typing the month name or a number above 12. The month field takes 1 to 12, so 13 or 0 returns nothing.
• Assuming every year divisible by 100 is a leap year. 1900 and 2100 are not, while 2000 and 2400 are, because they also divide by 400.
• Treating the result as the last working day. The tool gives the last calendar day, which may be a Saturday or Sunday, so check the weekday before using it as a deadline.
• Entering a two-digit year such as 24. The tool reads it as the year 24, not 2024, and gives the weekday for that ancient year.
• Entering the date format of another locale and swapping year and month. The date result is always year-month-day, as in 2024-02-29.