Date Calculator — Difference & Add/Subtract
Find the exact time between two dates — broken down into years, months, and days, plus total days, weeks, and an optional business-day count. Or flip to add/subtract mode to compute a date N days, weeks, months, or years away. Everything runs in your browser.
Private by design. All date math runs entirely in your browser — nothing you enter is sent to a server.
FreeNo sign-upRuns in your browserUtilitiesApplication
Why two date calculators disagree — the counting rules
Most "wrong" date results are not bugs, they are two defensible conventions. These are the four that cause nearly every discrepancy, and which one applies where.
| Question | Convention | Consequence |
|---|---|---|
| Is the end date included? | Exclusive by default (Mon → Fri = 4 days) | Inclusive counting adds 1 — contracts and hospital stays usually count inclusively |
| What is "one month later" from Jan 31? | Clamp to the last valid day (Feb 28/29) | Adding 1 month then subtracting 1 month may not return you to Jan 31 |
| How long is a "month" in days? | Calendar months, not a fixed 30 | A 6-month span is 181–184 days depending on which months it crosses |
| Do leap days count? | Yes — Feb 29 is a real day in any span crossing it | Year lengths alternate 365/366; "1 year" is not always 365 days |
- Leap years are every year divisible by 4, except centuries not divisible by 400 — 2000 was a leap year, 1900 was not, 2100 will not be.
- Business-day counts diverge further because holidays are jurisdiction-specific; a plain weekday count excludes only Saturday and Sunday.
- All arithmetic here runs on your local calendar in the browser, so a span that crosses a daylight-saving boundary is still counted in whole calendar days rather than 24-hour blocks.
How to calculate days between two dates
- Pick the mode. Choose "Days Between Dates" for a difference, or "Add / Subtract" to project a date forward or backward.
- Enter your dates. Use the date pickers. If the end date is before the start date, the tool swaps them automatically for the calculation.
- Read the breakdown. See the difference as years + months + days, total days, and full weeks. Tick "business days" to count only Mon–Fri.
- Copy the result. One click copies the summary (or the computed date in YYYY-MM-DD) to your clipboard.
Frequently Asked Questions
- Is anything I enter uploaded?
- No. All date math runs in your browser — nothing is sent to a server.
- Are both endpoints counted?
- The difference counts elapsed days: Jan 1 to Jan 2 is 1 day. To include both endpoints (inclusive counting), add 1 to the total.
- How are business days counted?
- Weekdays (Mon–Fri) after the start date through the end date. Public holidays are not excluded, since they vary by country and year.
- How does adding months handle month-end dates?
- Standard calendar arithmetic: Jan 31 + 1 month lands on Feb 28 (or Feb 29 in leap years) — the last valid day of the target month.
- Does it handle leap years?
- Yes. All calculations use real calendar dates, so Feb 29 and leap-year lengths are handled correctly.
- Can I calculate a deadline 90 days from today?
- Yes — use Add / Subtract mode with the +90 days quick chip, or type any amount in days, weeks, months, or years.
Use Cases
- Count days until a deadline, launch, or exam
- Compute contract or notice periods in business days
- Find your exact age or tenure in years, months, and days
- Project a 30/60/90-day follow-up date from today
- Check how many weeks are left in a project timeline
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