Days Between Dates Calculator

The calculator has six numeric fields arranged in two blocks: start date (day, month, year) and end date (day, month, year).

1. Type the day (1–31), the month as a number (1 = January, 12 = December) and the year (1900–2100) of your start date. 2. Do the same for the end date. 3. The result updates instantly: you get the total days, the full weeks, and the leftover days.

Typical uses in the U.S.: checking how many days are left on a 30-day return window, counting down to the closing date on a house, or tracking the 180 days of a visa stay. If the end date is earlier than the start date, the result is negative — it means the end date is already in the past. The count is a difference: from June 11 to June 12 there is 1 day, not 2 (the start day does not count itself).

The calculator converts each date into a serial day number (days elapsed since January 1, 1970) using the civil-calendar algorithm for the Gregorian calendar, then subtracts the two numbers. The formula in plain text:

`` year' = (month <= 2) ? year - 1 : year era = floor(year' / 400) yoe = year' - era * 400 mp = month + (month > 2 ? -3 : 9) doy = floor((153 * mp + 2) / 5) + day - 1 doe = yoe * 365 + floor(yoe / 4) - floor(yoe / 100) + doy serial = era * 146097 + doe - 719468 ``

Then: total days = serial(end) − serial(start); full weeks = trunc(days / 7); leftover days = days − weeks × 7.

Worked example for November 26, 2026 (Thanksgiving): November comes after February, so year' = 2026; era = floor(2026/400) = 5; yoe = 2026 − 2000 = 26; mp = 11 − 3 = 8; doy = floor((153·8 + 2)/5) + 26 − 1 = 245 + 25 = 270; doe = 26·365 + 6 − 0 + 270 = 9766; serial = 5·146097 + 9766 − 719468 = 20783. June 11, 2026 works out to serial 20615. Difference: 20783 − 20615 = 168 days, which is exactly 24 full weeks with 0 days left over. The shifted-year trick (starting the year in March) pushes the leap day to the very end, so the formula needs no month lookup tables.

The table shows four real calculations made with this tool:

Note the last row: 2024 was a leap year, so between February 28 and March 1 there are 2 days (February 29 counts). In a common year like 2026 it would be just 1. The Thanksgiving row shows a perfect week split: 168 = 24 × 7 + 0. And if you flipped the first row (July 4 back to January 1), the result would be −184: same interval, opposite sign.

This days-between-dates calculator gives you the exact answer instantly: enter a start date and an end date and you get the total number of days, the full weeks, and the leftover days. The math runs on the civil (Gregorian) calendar from 1900 to 2100, so leap years like 2024 and 2028 are counted correctly. Counting squares on a wall calendar is error-prone — it is remarkably easy to be off by one. Here the count is deterministic: from June 11 to November 26, 2026 there are exactly 168 days, which happens to be 24 full weeks on the dot. It also works backwards: if the end date comes before the start date, the result is negative, handy for figuring out how many days have passed since an event.