Example 1 — contract across a leap year
- Start: January 1, 2024
- End: January 1, 2025
- Include end date: off
366 days · 1 year at 365.25
A typical “from–to” contract through 2024. February 29 sits in the middle.
Date Difference Calculator
Enter a start date and an end date. The calculator counts calendar days between them on the Gregorian calendar. January 1, 2024 to January 1, 2025 is 366 days, because 2024 is a leap year. You can include the end date when the period is from–to inclusive — for example leave from July 1 through July 14 then gives 14 days instead of 13. The result does not skip weekends or holidays: these are not business days.
Result
Enter both dates on the left. Counting elapsed days (end date not added). The result shows up here.
Age to today: age from date of birth. Date in N days: add days to date. Weekdays only: business days.
Worth knowing
A leap year makes a “one-year” contract one day longer
When a U.S. lease, warranty, or internship is written as from–to dates instead of a ready day count, a leap day counts like any other calendar day. January 1, 2023 to January 1, 2024 is 365 days. January 1, 2024 to January 1, 2025 is 366, because February 29, 2024 falls in the middle.
That is not a calculator quirk. It is how the Gregorian calendar works: the mean year is about 365.25 days, and a full leap year has 366. Including the end date is a separate choice — common for vacation and event planning in the U.S. — and adds one more day on top of elapsed time.
“January 1, 2024 to January 1, 2025 is 366 days. The same one-year-later label without February 29 is 365.”
How it works
StartJan 1, 2024
EndJan 1, 2025
Days366
The result is the number of calendar days from the start date to the end date, not the gap between two clock times. January 1, 2024 to January 1, 2025 is 366 days, because 2024 has February 29. January 1, 2023 to January 1, 2024 is 365 days. The same “one year later” label does not always mean the same number.
Late February makes that even clearer. February 28, 2024 to March 1, 2024 is 2 days. February 28, 2023 to March 1, 2023 is 1 day. The calculator does not round months to 30 days and does not skip weekends or holidays.
Beside the result you also see “years at 365.25”: days divided by 365.25 and rounded down. 365 divided by 365.25 is a fraction below 1, so a common year with no February 29 comes out as 0 of those years. 366 days in a leap year give 1. That is a rough Gregorian year length, not age in birthdays.
When the start date is later than the end date, the day count is negative. January 1, 2025 minus January 1, 2024 is −366. That is the same formula the other way. When both dates are the same, the result is zero. Swap dates reverses the fields without retyping.
The fields are calendar days. The calculator does not ask for a clock time or a time zone. Turning the clock on the last Sunday in March does not shrink a weekend in this count. The US units switch does not change anything here: days are not miles or pounds.
People often use this to check how many days a contract or internship runs when the document only has dates; whether February 29 falls in the period; or how many whole years at 365.25 fit before an addendum. If one date should always be today and the other is a birth date, open the age calculator. When you know a start date and a day count and want the date after adding those days, open add days to a date.
How to use
- In “1. Start date”, pick the start — for example the first day of a lease, internship, or warranty.
- In “2. End date”, pick the end. Order matters: reversed dates give a negative result.
- Check “Include end date” when the period is from–to inclusive (leave, an event). Leave it off for plain elapsed time.
- Read calendar days on the first row. Beside that are years–months–days, years at 365.25, weeks, and how many times February 29 falls in the period.
- The timeline above the results shows the start, the end, and any February 29. Copy summary puts ready text on the clipboard.
- Age to today is on the age page. A date after N days is on add days. Weekdays only — on business days.
Formula
elapsed days = end date − start date (UTC civil days)
days with end included = elapsed days + 1 when Include end date is on
years, months, days = calendar walk from the earlier date to the later one (like subtracting a date on paper)
years365.25 = floor(days / 365.25)
365 days give 0 years at 365.25. 366 days in a leap year give 1. The February 29 count is how many leap days fall strictly after the start and on or before the end.
Civil days, inclusive end, and 365.25
Elapsed days are the end date minus the start date. January 1, 2024 to January 1, 2025 is 366. Years–months–days follows the calendar; years at 365.25 is a separate measure (days divided by 365.25 and rounded down).
- civil days
- A calendar gap, not hours. February 28, 2024 to March 1, 2024 is 2, because February 29 falls in the middle.
- include end date
- Adds 1 to elapsed days. Leave from July 1 through July 14 inclusive is 14 days, not 13.
- years–months–days
- A calendar breakdown, like subtracting a date on paper. Not the same as years at 365.25.
- February 29
- How many leap days fall in the period. A contract through 2024 shows 1.
Real-life examples
The button fills both dates and recalculates the same way as entering them by hand.
Example 2 — common year, no February 29
- Start: January 1, 2023
- End: January 1, 2024
- Include end date: off
365 days · 0 years at 365.25
365 divided by 365.25 is below 1, so rounding down gives 0 years at 365.25. The calendar breakdown is still 1 year.
Example 3 — late February in a leap year
- Start: February 28, 2024
- End: March 1, 2024
- Include end date: off
2 days · 0 years
February 29 sits in the middle. In 2023 the same layout is 1 day.
Example 4 — late February without February 29
- Start: February 28, 2023
- End: March 1, 2023
- Include end date: off
1 day · 0 years
February 2023 ends on the 28th. The same “end of February” label is not always the same length.
Example 5 — four years with one leap day
- Start: January 1, 2020
- End: January 1, 2024
- Include end date: off
1,461 days · 4 years at 365.25
2020 is a leap year. 1,461 ÷ 365.25 rounded down is 4 years.
Example 6 — reversed dates
- Start: May 15, 1990
- End: August 8, 1988
- Include end date: off
−645 days · −2 years at 365.25
End before start. The absolute value is 645 days.
Example 7 — leave from–to inclusive
- Start: July 1, 2024
- End: July 14, 2024
- Include end date: on
14 days · 0 years at 365.25
Without including the end it would be 13. Turning the switch on adds 1 day to the elapsed gap.
Where this helps
- You check how many days a job contract, freelance agreement, or internship runs when the text has start and end dates, not a ready count.
- You compare a warranty or insurance period with an addendum and see whether February 29 falls in the middle.
- You measure the gap between two events (for example registration and pickup) before you put it in a note or a claim.
Frequently asked questions
How does the calculator count days between two dates?
It subtracts the start date from the end date on the Gregorian calendar in UTC. The result is calendar days that have elapsed. January 1, 2024 to January 1, 2025 is 366, because 2024 has February 29.
When should I turn on Include end date?
When the period is from–to inclusive, for example leave from July 1 through July 14. The calculator then adds 1 day to the elapsed gap. For plain elapsed time, as with age, leave the switch off.
How do years–months–days differ from years at 365.25?
Years–months–days is a calendar breakdown, like subtracting a date on paper. Years at 365.25 is days divided by 365.25 and rounded down. For January 1, 2023 to January 1, 2024 (365 days) that measure gives 0 years, while the calendar breakdown is 1 year.
Does Saturday or a holiday drop out of the result?
No. Every calendar day counts the same. Weekends and holidays stay in the result. Business days without Saturday and Sunday belong on the business-days calculator.
What does the February 29 count mean?
How many times February 29 falls strictly after the start date and on or before the end date. A contract from January 1, 2024 to January 1, 2025 has one such day.
What if the start date is after the end date?
The day count comes out negative, and the formula is the same both ways. Swap dates reverses the fields without retyping.
Is this age from date of birth?
No. Here you enter both dates yourself. Age from date of birth counts years, months, and days to today or to a chosen day another way.
Where do I add N days to one date?
On the add-days-to-a-date page. Here you already have both dates and read the gap between them.
What practical problems does this calculator solve?
Contracts, internships, warranties, inclusive leave ranges, and gaps between two events when a document has dates, not a ready day count.
Does the US units switch change the result?
No. Years and days are not miles or pounds. The metric / US switch does not convert anything here.
How many days from January 1, 2024 to January 1, 2025?
366 calendar days, because 2024 has February 29. Without including the end date the result is the elapsed gap, not a paper “one year” label.
How many days is leave from July 1 through July 14 inclusive?
With include end date on, the result is 14 days. Without the switch the elapsed gap is 13 days.
Are weekends and holidays subtracted?
No. The calculator counts every calendar day. Business days with holidays are not computed on this page.
What does years at 365.25 mean?
It is floor(days / 365.25): an approximation of the Gregorian year length. 365 days gives 0 such years; 366 days in a leap year gives 1. The years–months–days line is a separate calendar breakdown.
What if the end date is earlier than the start date?
The result is negative: the same day count with a minus sign. You can swap the dates with the swap control.
Is February 28, 2024 → March 1 the same as in 2023?
No. In 2024 the result is 2 days (February 29 is in between). In 2023 the same date pair gives 1 day.
Knowledge sources
Day counts come from the Gregorian calendar. This is date subtraction, not a time-zone tool. 365.25 is the classic mean year length in that calendar.
Page updated November 3, 2026.