Does the calculator count the start and end date, or just one?

For days-between it shows both: exclusive (default, end date not counted) and inclusive (both counted). The off-by-one between these is the most common date-math mistake, so the tool always displays them side by side so you pick the one you need.

Does it handle leap years and different month lengths?

Yes. The years/months/days breakdown uses real calendar arithmetic (the DATEDIF borrow method), so leap years and 28–31 day months are handled exactly. Approximate months and years derived from total days ÷ 30.44 and ÷ 365.2425 are labeled as estimates.

Is the date calculator free and private?

Yes — 100% free. Everything is calculated in your browser; no dates are transmitted or stored. 'Today' is read from your device clock, so an incorrect system clock would give an incorrect 'today'.

Date Calculator

Q: How do I calculate the number of days between two dates?

Pick a start date and an end date and the calculator subtracts one from the other. By default it counts days exclusively (the start date counts, the end date does not), which matches Excel DAYS and timeanddate.com. The inclusive value (both endpoints counted) is always shown alongside, and inclusive = exclusive + 1.

Q: What date is 90 days from today?

Switch to Add / Subtract mode, leave the base date on today, set the amount to 90 days and direction to After. The tool returns the exact date and the weekday it lands on. You can also add weeks, months, or years, or count only business days (Mon–Fri).

Q: How many business days are between two dates?

Set the day-counting basis to Business days only (Mon–Fri). The calculator follows NETWORKDAYS rules: it counts both endpoints when they are workdays, skips Saturdays and Sundays (or a custom weekend set), and subtracts any holidays you enter that fall on a weekday inside the range.

Q: What happens when I add a month to January 31?

Adding one month to January 31 lands on the last valid day of February — February 28, or February 29 in a leap year — not March 3. The tool clamps the day-of-month to the end of the target month and tells you when it rolled to month end, avoiding the common JavaScript date-overflow bug.

Days between dates, add or subtract days, and count down to an event — with optional business-day & holiday handling.

Start date

The earlier endpoint. Defaults to today.

End date

The second endpoint. Pick a date to compute.

Day-counting basis

Choose "business days" to skip weekends (and any holidays you enter).

Endpoints

Headline uses this; both values are always shown.

Result

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Date Calculator — guide & how to use

Calendar dates, not timestamps. This tool counts calendar dates — time-of-day, hours, and time zones are ignored, and "days between" means a count of date boundaries, not 24-hour periods. All math runs on UTC-midnight day serials specifically to avoid the Daylight Saving off-by-one errors that happen when you subtract two local-time dates across a clock change. The Gregorian calendar is assumed (results before its 1582/1583 adoption are not historically accurate). Everything is calculated in your browser — no dates are sent anywhere — and "today" is read from your device clock, so a wrong system clock yields a wrong "today".

Three modes, one tool

Difference between two dates (default) — how long from a start date to an end date, as a years/months/days breakdown and a total-days count.
Add / subtract from a date — "what date is 90 days from today?" or "what date was 6 months ago?"
Days until / since an event — a countdown (or count-up) from today to a target date.

Worked example

What date is 90 days after June 9, 2026? In Add / subtract mode with amount 90, unit days, direction After:

Day serial of 2026-06-09 plus 90 days lands on September 7, 2026 — a Monday.
Switch the basis to business days and "90 business days after" instead steps over weekends, counting only Mon–Fri (and skipping any holidays you enter), so it lands much later in the year.

A difference example: from 2024-01-15 to 2026-03-29 the calendar breakdown is 2 years, 2 months, 14 days, while the total is 804 days exclusive (805 inclusive).

How to use it

Pick a mode at the top. The inputs change to match it.
Choose your dates with the native date pickers (a 4-digit year avoids 26-vs-2026 ambiguity).
In Add / subtract mode, set the amount, unit, and direction.
Set the day-counting basis — all calendar days, or business days (Mon–Fri).
For business days, open Advanced to change the weekend definition or paste holidays to exclude.
Read the headline plus the full breakdown, then Copy result or Copy share link.

How the math works

Every date is converted to an integer day serial via Date.UTC(y, m-1, d) / 86400000. Calendar days between two dates is simple integer subtraction of serials, so it is exact and DST-proof. The exclusive count (Excel DAYS convention) does not count the end date; the inclusive count adds 1. The years/months/days breakdown uses the DATEDIF borrow method on real calendar dates, so leap years and unequal month lengths are handled exactly. Adding months/years clamps the day-of-month to the end of the target month (Jan 31 + 1 month → Feb 28/29), and business-day counting follows NETWORKDAYS: both endpoints count when they are workdays, weekends and weekday-holidays are skipped.

Accuracy notes & assumptions

Calendar dates only — no time-of-day, hours, or time zones. "Days between" counts date boundaries.
UTC-midnight serials are used on purpose to avoid DST off-by-one errors from subtracting local-time dates.
Default is exclusive of the end date (matches Excel DAYS and timeanddate.com). The inclusive value is always shown; inclusive = exclusive + 1.
Business days follow NETWORKDAYS / NETWORKDAYS.INTL: weekends and supplied holidays are excluded, and both endpoints are counted when they are workdays.
Month/year math uses real calendar rules (leap years, 28–31 day months). The "approx. months/years" figures from total-days ÷ 30.44 / ÷ 365.2425 are estimates and labeled as such.
Holidays are user-supplied only — no built-in holiday calendar, so business-day results are accurate only for the holidays you enter.
Gregorian calendar assumed; pre-1583 results are not historically accurate, and years before 1583 are flagged.
All computation is client-side; no dates are transmitted or stored.

Common mistakes this tool prevents

Inclusive vs exclusive off-by-one — the #1 date-math error. Both values are always shown, labeled.
Subtracting two local Date objects across a DST change (yielding 89.96 or 90.04 days). UTC-midnight serials avoid it.
Treating a month as 30 days or a year as 365 over long spans. The Y/M/D breakdown is the source of truth.
The setMonth() overflow bug (Jan 31 + 1 month → March 3). The tool clamps day-of-month to the month end.
Counting a weekend holiday as an extra business-day exclusion (double-counting). Only weekday holidays are subtracted.
Assuming "business days from today" counts today — WORKDAY starts the day after the base date, and this tool follows that.

How to embed this tool on your website

No account, no coding. Copy the embed code and paste it where you want it to appear:

<iframe src="https://snaptoolsuite.com/date-calculator/?embed=1"
  style="width:100%;max-width:560px;height:760px;border:0;"
  title="Date Calculator" loading="lazy"></iframe>
<p>Free <a href="https://snaptoolsuite.com/date-calculator/">Date Calculator</a> by Snap Tool Suite</p>

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Frequently asked questions

How do I calculate the number of days between two dates?

Pick a start and end date; the tool subtracts the serials. By default it counts exclusively (end date not counted, matching Excel DAYS and timeanddate.com) and always shows the inclusive value too — inclusive = exclusive + 1.

What date is 90 days from today?

Use Add / subtract mode, leave the base on today, set amount 90, unit days, direction After. You get the exact date and the weekday it lands on. You can also add weeks, months, years, or count only business days.

How many business days are between two dates?

Set the basis to Business days only. The tool follows NETWORKDAYS: it counts both endpoints when they are workdays, skips weekends (or your custom weekend set), and subtracts weekday holidays you enter.

Does it count the start date, the end date, or both?

For days-between it shows both: exclusive (default, end not counted) and inclusive (both counted). They differ by exactly one, so you can pick whichever your situation needs.

What happens when I add a month to January 31?

It clamps to the last valid day of February (28, or 29 in a leap year), not March 3, and tells you it rolled to month end — avoiding the JavaScript date-overflow bug.

Does it handle leap years and uneven months?

Yes — the years/months/days breakdown uses real calendar arithmetic (DATEDIF borrow method), so leap years and 28–31 day months are exact. The approximate months/years are labeled estimates.

Is it free and private?

Yes. 100% free, and everything runs in your browser — no dates leave your device.

Related tools

Add this calculator to your site

Free to embed on any website, blog, or client portal. One line of HTML — no account, no code, no cost.

<iframe src="https://snaptoolsuite.com/date-calculator/?embed=1"
  style="width:100%;max-width:560px;height:760px;border:0;"
  title="Date Calculator" loading="lazy"></iframe>

2 Paste it on your page

3 It just works

Everything is calculated in your browser. We don't store or sell your data.