Days between dates: decide what you are counting

Elapsed days versus included dates

September 1 to September 7 is a six-day interval: six midnight boundaries separate the dates. An event that includes both September 1 and September 7 covers seven dates. Our default is the interval. Selecting “Include both start and end dates” adds one to the result.

Using the same date twice produces zero by default and one with both endpoints included. This distinction helps with trip and event planning, but the tool does not interpret contracts or deadlines governed by special rules. Check those rules separately.

Why age is not days divided by 365

Years do not all have 365 days. Under the Gregorian rule, a year divisible by 4 is normally a leap year, except years divisible by 100 but not by 400. Consequently, 2000 was a leap year and 2100 will not be.

We count completed birthday anniversaries rather than dividing elapsed days by 365. For February 29 birthdays we use February 28 in a non-leap year. This is an explicit convention for the age and display-span calculations, not a claim about every legal or cultural use of birthdays.

Calendar dates are not durations in hours

We encode the year, month, and day as a UTC-midnight marker and subtract those markers to count calendar days. This does not mean you were born at midnight UTC. It prevents daylight-saving transitions or the browser time zone from turning a calendar interval into a fractional result because a local day lasts 23 or 25 hours.

On first load, the reference date is set to your device’s local date. You can change it. If midnight passes with the page open, your chosen date is left alone; update it or refresh when needed. This tool does not calculate hours, time-zone conversions, public holidays, or working days.

Boundary examples to check yourself

February 28 to March 1, 2024 is two elapsed days, or three included dates. The same month and day pair in 2023 is one elapsed day. January 1 to December 31, 2026 is 364 elapsed days, or 365 dates with both endpoints included.

Inputs use the Gregorian calendar and accept valid dates from 1900 onward. An end date before a start date produces an error instead of silently swapping the dates. A birth date after the reference date is also rejected, making a mistyped year easier to spot.

If your result differs

Check the endpoint option, reference date, and whether you are comparing completed years with completed weeks. If a result still appears incorrect, email a reproducible pair of example dates and the expected result using the contact link. A fictional birthday is enough; you do not need to disclose personal information.

This explanation is maintained alongside the calculation logic. For background on calendar rules, see the U.S. Naval Observatory’s Calendars page; for the date representation used in software, see MDN’s Date documentation.

Reference reading

• U.S. Naval Observatory — Calendars
• MDN — JavaScript Date