📅 Date Calculator
Date Add / Subtract
Date Difference
What This Tool Does
This page is really two tools. Date Add/Subtract answers "what date is X years, months, and days from a given date?" Date Difference answers "how far apart are these two dates?" One projects a new date from a starting point; the other measures the gap between two known dates. Mixing these two questions up is the most common source of date-math mistakes.
Everything runs in your browser using standard Gregorian calendar rules — leap years, unequal month lengths, and end-of-month rollovers are handled for you, with no spreadsheet formulas required.
Counting Days: Inclusive vs. Exclusive
"How many days from January 1 to January 3?" has two defensible answers: 2 (exclusive — count the nights, like a hotel stay) or 3 (inclusive — count every calendar day, like a rental or a course of medication). This is the classic fencepost problem, and it's why the tool has an Include End Date checkbox:
- Leave it unchecked for elapsed time: countdowns, ages, "days since" — and note that spreadsheet functions like
DATEDIFcount this way too. - Check it when both endpoints count: billable days, prescription durations, event lengths, most lease and insurance terms.
When a contract deadline matters, verify which convention the document uses — "within 30 days of receipt" almost always means exclusive counting starting the day after receipt.
Why "One Month Later" Is Ambiguous
Months are 28–31 days long, so "add one month" is calendar arithmetic (bump the month, keep the day) while "add 30 days" is plain day counting. They frequently disagree:
| Base date | + 1 month | + 30 days |
|---|---|---|
| Jan 15, 2026 | Feb 15, 2026 | Feb 14, 2026 |
| Jan 31, 2026 | Feb 28, 2026 (clamped to month end) | Mar 2, 2026 |
| Jan 31, 2024 (leap year) | Feb 29, 2024 | Mar 1, 2024 |
When month arithmetic lands on a day that doesn't exist (February 31st), this tool clamps to the last day of the target month — the same behavior as most programming languages, banks, and billing systems. For "net 30" invoices or any "within N days" clause, use the days field, not the months field.
Leap Years in 30 Seconds
A year is a leap year if it's divisible by 4 — except century years, which must be divisible by 400. So 2024 and 2000 are leap years, but 1900 and 2100 are not. The exception exists because a solar year is about 365.2422 days, slightly less than 365.25; skipping three leap days every 400 years keeps the calendar aligned with the seasons.
This tool applies the full rule automatically, including the tricky case of projecting February 29 into a non-leap year (it clamps to February 28).
Handy Offsets, Ready to Plug In
- Net-30 / net-60 invoices: add 30 or 60 days to the invoice date for the payment due date.
- 90-day visa or visa-waiver windows: add 90 days to your entry date; most immigration authorities count the entry day itself, so double-check whether to subtract one.
- Pregnancy due date: add 280 days (40 weeks) to the first day of the last menstrual period — Naegele's rule.
- Probation and notice periods: add 3 or 6 months to a start date, minding the month-end clamping explained above.
- Return windows: "30 days from delivery" is day arithmetic — use the days field.
Frequently Asked Questions
Why do the months/years rows in the difference result show decimals?
Those rows convert the total day count using average lengths (30.44 days per month, 365.25 days per year) — useful for a quick sense of scale. The Total row underneath is the exact calendar breakdown (X years, X months, X days); use that one for anything official.
Do time zones affect the results?
No. This is pure calendar math on whole dates — no clock times are involved, so there's nothing for a time zone to shift. If you need hour-level differences across time zones, use our Time Calculator or World Time Hub instead.
Can I use historical or far-future dates?
Yes — any date the Gregorian calendar can express. One caveat for historians: dates before a country adopted the Gregorian calendar (1582 in Catholic Europe, 1752 in the British Empire) were recorded in the Julian calendar, and this tool does not convert between the two.
How do I count business days instead of calendar days?
This tool counts calendar days. As a rule of thumb, a full week contributes 5 business days, so divide the day count by 7 and multiply by 5 for an estimate — but for deadlines that legally depend on business days, count the specific weekdays and public holidays involved.