Date Calculator
Use this date calculator to add or subtract days, weeks, months or years from any date.
Jan 26, 2027 is a Tuesday in ISO week 4, a working day (weekends only are checked).
On the calendar
Start: Wed, Oct 28, 2026. Result: Tue, Jan 26, 2027, 90 days later.
Is it a working day?
- Result day
- Tuesday, working day
- Working day before
- Mon, Jan 25, 2027
- Working day after
- Wed, Jan 27, 2027
- Working days in the span
- 64
Counted Monday to Friday after the earlier date, up to and including the later one.
US federal holidays with observed dates (5 U.S.C. 6103); England and Wales bank holidays with substitute days (GOV.UK).
Working
- 1Start dateWed, Oct 28, 2026
- 2Add weeks and days as exact daysOct 28, 2026 + 90 days = Jan 26, 2027
- 3Check: count the days between the two datesJan 26, 2027 − Oct 28, 2026 = 90 days
Find the offset to a date
- Days
- 58 after
- Weeks and days
- 8 weeks, 2 days
- Calendar months
- 1 month, 27 days
Repeat the offset
| Days | ISO week | Repeat | Note | |
|---|---|---|---|---|
| Tue, Jan 26, 2027 | 90 | 4 | ×1 | |
| Mon, Apr 26, 2027 | 180 | 17 | ×2 | |
| Sun, Jul 25, 2027 | 270 | 29 | ×3 | Weekend |
| Sat, Oct 23, 2027 | 360 | 42 | ×4 | Weekend |
| Fri, Jan 21, 2028 | 450 | 3 | ×5 | |
| Thu, Apr 20, 2028 | 540 | 16 | ×6 |
Each date is 90 days times the repeat number, counted from Oct 28, 2026, so monthly dates keep their day of the month after a short month.
Common offsets
| Offset | Date |
|---|---|
| +7 days | Wed, Nov 4, 2026 |
| +14 days | Wed, Nov 11, 2026 |
| +30 days | Fri, Nov 27, 2026 |
| +1 month | Sat, Nov 28, 2026 |
| +60 days | Sun, Dec 27, 2026 |
| +90 days | Tue, Jan 26, 2027 |
From Oct 28, 2026, going forward. Compare 90 days with 3 months: they rarely land on the same date.
Apply the offset to a list of dates
Paste a column of start dates from a spreadsheet, as 2026-10-30 or 10/30/2026. Each row gets the date 90 days after it.
| # | Start date |
|---|---|
| 1 | |
| 2 | |
| 3 | |
| 4 |
Results
| Start | Result | Note |
|---|---|---|
| Sep 30, 2026 | Tue, Dec 29, 2026 | |
| Oct 15, 2026 | Wed, Jan 13, 2027 | |
| Oct 30, 2026 | Thu, Jan 28, 2027 | |
| Nov 13, 2026 | Thu, Feb 11, 2027 |
How it's worked out
You give a start date S and an offset of Y years, M months, W weeks and D days. The tool adds the calendar units first and the exact units second:
T is the date after the months, and R is the result. Months and years keep the day of the month and move the month forward. Weeks and days are counted one day at a time, with every leap day included.
A month is not a fixed number of days, so the tool needs a rule for a day that does not exist. It uses the last day of the month. January 31, 2026 plus one month is Feb 28, 2026. In the leap year 2024 the same step gives Feb 29, 2024.
The order matters too. January 31, 2026 plus one month and one day is Mar 1, 2026: first the month (to February 28), then the day. Most people read "one month and one day" this way, and so does most software.
The start date is day 0. Add 1 day to a Monday and you get Tuesday. Subtract works the same way, going back.
Worked examples
Maria's net-30 invoice
Maria sends an invoice on Friday, October 30, 2026, with payment due in 30 days. Thirty days later is Sun, Nov 29, 2026. Her client's bank only pays on working days, so the money arrives on Mon, Nov 30, 2026. The Is it a working day card shows this next working day for any result.
Leo's 6-month probation
Leo starts a new job on Monday, August 31, 2026, with a 6-month probation. February 2027 has no 31st, so the month-end rule gives Sun, Feb 28, 2027. That is 181 days. If his contract says "182 days" instead of "6 months", the date is different, so read the wording.
Aisha's visa deadline
Aisha's conference opens on Monday, March 15, 2027. Her visa form must go in 8 weeks before. Subtract 8 weeks and you reach Mon, Jan 18, 2027. That day is Birthday of Martin Luther King, Jr., a US federal holiday, so offices are closed. With US federal holidays switched on, the tool points her to Fri, Jan 15, 2027 instead.
Reading your result
The big date is the answer. Under it you get the weekday and the number of calendar days from the start. The tiles add the ISO week and the day of the year. ISO weeks start on Monday, and week 1 is the week with the year's first Thursday (ISO 8601). Many offices in Europe plan by these week numbers.
If the date falls on a Saturday or Sunday, the sentence under the answer says so and names the working days on either side. Pick US federal or England and Wales holidays and the tool checks those too. US federal holidays follow 5 U.S.C. 6103, with a Saturday holiday observed on Friday and a Sunday one on Monday. England and Wales bank holidays move to the next weekday when they fall on a weekend.
Days and months answer different questions. From November 2, 2026, 90 days is Jan 31, 2027 but 3 months is Feb 2, 2027. A rule written in days means exact days. A rule written in months means the same day number, or the month's last day.
Common mistakes
- Counting the start day as day 1. "Within 10 days" of the 1st usually means the 11th. If your rule counts the first day, enter one day less.
- Using 365 days for a year. From June 1, 2027, 365 days is May 31, 2028, because the span holds February 29, 2028. One year is Jun 1, 2028.
- Forgetting that a deadline can move. Courts, tax offices and banks often push a weekend or holiday deadline to the next working day. Others pull it back. Check the rule, then use the working day card.
- Adding months as 30 days. Six months is not 180 days. It can be 181 to 184 days, depending on which months you cross.
- Mixing time zones near midnight. The tool works in whole calendar days. Enter the date in the time zone where the deadline applies.
Questions people ask
Sources
- Dershowitz, N. and Reingold, E. M. Calendrical Calculations, 4th ed. Cambridge University Press (Gregorian date arithmetic)
- ISO 8601-1:2019, Date and time representations (week numbering)
- 5 U.S.C. 6103, Holidays
- GOV.UK, UK bank holidays
Maintained by Anil ShahUpdated October 28, 2026Checked against 14 reference values