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Count the days between two dates, or add/subtract days from any date.
Dates are converted to millisecond timestamps; the difference divided by 86,400,000 (ms per day) gives the day count. Adding days uses calendar arithmetic, so month lengths and leap years are handled automatically. Worked example: Jan 1, 2026 to Dec 31, 2026 = 364 days (2026 is not a leap year).
365 normally, 366 in a leap year. Leap years are divisible by 4, except centuries not divisible by 400 (so 2000 was a leap year, 1900 was not).
This calculator counts the difference between the two dates — the number of midnights from one to the other, which is the standard convention.
Enter today and 90 in the second tool above — it handles month and year rollover for you.