What does the Date/Time Calculator do?
The Date/Time Calculator does the two jobs people actually open a time and date calculator for. Duration mode tells you how much time sits between two dates: years, months, weeks and days, plus the totals in days, hours, minutes and seconds. Add / Subtract mode takes a date, moves it forward or back by any mix of years, months, weeks and days, and tells you the date you land on, with the weekday.
The calendar is a bad thing to do arithmetic on in your head. Months run 28 to 31 days. Years run 365 or 366. "Three months from now" and "90 days from now" are different dates. Counting squares on a wall calendar works until you skip a row. The calculator does the counting and shows its answer in the units you need for the form in front of you.
There's no Calculate button, no account and nothing to install. Results update as you change a date, and the math runs in your browser.
How to use it
Find the time between two dates
- Stay on Duration between dates, the tab the page opens on.
- Pick a start date and an end date. Both start on today.
- Read the summary line, such as
0y 2mo 4w 1d (90 total days), and the eight boxes under it: Years, Months, Weeks, Days (remainder), Total days, Total hours, Total minutes and Total seconds.
The order doesn't matter. Put the later date first and you get the same answer, not a negative one.
Add or subtract time from a date
- Switch to Add / Subtract time.
- Pick the base date and choose + Add or − Subtract.
- Fill in any of Years, Months, Weeks and Days. Leave the rest blank.
- Read the result date, written out with its weekday, plus the same date in
YYYY-MM-DDform.
Every number has a copy button: the summary, each box, the written date and the ISO date. That's handy when the answer is going into a spreadsheet, a contract or a calendar invite.
Worked examples with real dates
Here's what the calculator returns for the questions people search for most. Each one was run through the calculator's own code.
| Question | Enter | Result |
|---|---|---|
| 90 days from May 13, 2026 | May 13, 2026 + 90 days | Tuesday, August 11, 2026 |
| 3 months from May 13, 2026 | May 13, 2026 + 3 months | Thursday, August 13, 2026 |
| 6 months from May 13, 2026 | May 13, 2026 + 6 months | Friday, November 13, 2026 |
| 45 days before June 1, 2026 | June 1, 2026 − 45 days | Friday, April 17, 2026 |
| 40 weeks from October 13, 2026 | October 13, 2026 + 40 weeks | Tuesday, July 20, 2027 |
| 1 month, 2 weeks and 3 days from March 15, 2026 | March 15, 2026 + 1 mo, 2 wk, 3 d | Saturday, May 2, 2026 |
| The 100th day of 2026 | January 1, 2026 + 99 days | Friday, April 10, 2026 |
| The 100th day of 2028 (a leap year) | January 1, 2028 + 99 days | Sunday, April 9, 2028 |
The 100th-day rows add 99, not 100, because January 1 is already day 1. That's the same off-by-one that trips up most hand counts, and it's worth a closer look.
Duration example: a 90-day contract window
A contract signed on May 13, 2026 promises delivery within 90 days. Put May 13, 2026 as the start and August 11, 2026 as the end. The summary reads 0y 2mo 4w 1d (90 total days). The boxes show 2,160 total hours, 129,600 total minutes and 7,776,000 total seconds.
Read that breakdown as "2 months, then 4 weeks and 1 day." May 13 to July 13 is two calendar months. July 13 to August 11 is 29 days, which is 4 weeks and 1 day. The weeks figure is carved out of the leftover days, so it never double-counts the months.
Does it count the start date and the end date?
It counts the gap between them, the way a ruler measures from 0. January 1 to January 2 is 1 day. May 13 to August 11 is 90 days. The start date is day zero and the end date is where you stop.
Some questions want both ends counted. A hotel stay from the 3rd to the 5th is 2 nights, but a conference running the 3rd to the 5th is 3 days. A medication course "from the 1st to the 10th, inclusive" is 10 days. When both ends count, add 1 to the total days.
Rule of thumb: nights, rent and interest count the gap. Event days and "inclusive" ranges count both ends, so add 1. If a contract says "within 90 days of signing," the gap count is almost always what it means. If it matters, read the clause; the calculator can't read it for you.
Months are the tricky unit
Days and weeks are fixed lengths. Months aren't, and that's where every date calculator has to make a choice.
"3 months" is rarely 90 days
Adding 3 months keeps the day of the month: May 13 becomes August 13. That span is 92 days, because May (18 days left after the 13th), June (30), July (31) and August (13) add up to 92. November 13, 2026 to February 13, 2027 is also 92 days. So "3 months" and "90 days" from the same date land two days apart. If a deadline says one, use that one.
Month-end dates stop at the end of the month
When the target month is too short for the day you started on, the calculator lands on that month's last day. It never spills over into the month after.
- January 31, 2026 + 1 month = February 28, 2026.
- January 31, 2028 + 1 month = February 29, 2028 (a leap year).
- March 31, 2026 + 1 month = April 30, 2026.
- February 29, 2024 + 1 year = February 28, 2025 (2025 has no February 29).
Duration mode counts months the same way, so January 31 to February 28, 2026 is exactly 1 month, and January 31 to March 1, 2024 is 1 month and 1 day. Dates from the 1st to the 28th are never affected. If a contract means "30 days," add 30 days instead of a month.
Leap years and long spans
A year is a leap year if it divides by 4, unless it divides by 100, unless it also divides by 400. So 2024 and 2028 are leap years, 2100 won't be, and 2000 was. The calculator follows that rule without you doing anything, which is why it reports 3,653 days from January 1, 2020 to January 1, 2030: ten years of 365 days plus the leap days in 2020, 2024 and 2028.
Long spans are where the year-month-day breakdown earns its place. Someone born on June 15, 1990 was 36y 3mo 0w 0d (13,241 total days) old on September 15, 2026. If birthdays are your main use, the Age Calculator frames the same math around a birth date, and Age Difference Calculator compares two people.
The calculator works from the year 100 onward. For dates before 1582, remember the answer is on the modern Gregorian calendar, which isn't the calendar people were using at the time.
What it doesn't do (and what does)
This is a calendar-date calculator. It's clear about where that stops.
- Clock times. You pick dates, not times, so there's no "add 36 hours to 9:30 PM." For the hours and minutes between two clock times, use the Hours Calculator. For a time in another city, the Time Zone Converter.
- Business days. Weekends and holidays count like any other day. To skip weekends and US holidays, use the Business Days Calculator.
- Fractions of a unit. The add fields take whole numbers. Type 1.5 months and it adds 1. For a month and a half, enter 1 month and 15 days.
- "From today" in one step. Both modes start on today's date, so it works. If counting forward from today is all you need, the Days From Today Calculator gets there with one number.
Most date tools online come wrapped in a calendar product that wants your email, or in a page with four ad slots between you and the answer. This is the calculator, the result and a copy button.
Frequently asked questions
Does the order of the two dates matter?
No. Duration mode always measures from the earlier date to the later one, so swapping them gives the same result. You won't get a negative number of days.
Should I count the end date?
The calculator doesn't. It measures the gap, so the 1st to the 2nd is 1 day. If your question counts both the first and last day, such as event days or an "inclusive" range, add 1 to the total.
Is 3 months the same as 90 days?
Almost never exactly. Three months from May 13, 2026 is August 13, which is 92 days. Ninety days from May 13, 2026 is August 11. Use whichever unit the deadline is written in.
What happens when I add a month to January 31?
You get the last day of February: February 28 in 2026, February 29 in a leap year like 2028. The calculator stops at the end of a short month rather than carrying the extra days into March.
Can I add hours and minutes to a date?
No. The inputs are calendar dates, and the add fields are years, months, weeks and days. The duration totals are given in hours, minutes and seconds, counted from midnight to midnight. For the time between two clock times, use the Hours Calculator.
Can it skip weekends and holidays?
Not this one. It counts calendar days, which is also how most "within X days" deadlines are written. The Business Days Calculator counts weekdays only and skips US holidays.