How the three modes work
Most date tools do one thing. This one covers the three jobs that come up in practice, and it shows the reasoning behind every number it returns.
- Days between dates measures a span. You get the raw day count, a weeks-and-days breakdown, the calendar equivalent in years, months and days, the number of whole months, and how many of those days are actually working days.
- Add or subtract moves a date forward or backward by days, weeks, months or years. This is the mode you want for payment terms, notice periods, warranty windows and subscription renewals.
- Business days removes Saturdays and Sundays from a range, and optionally the eleven US federal holidays as well. You can count working days across a span or push a start date forward by a number of working days to find a delivery or response deadline.
The rules behind the numbers
Counting the span
The total is the end date minus the start date, expressed in whole days. The start day itself is not counted and the end day is. That is the convention banks, payroll systems and most contracts use, and it is why the 1st to the 31st of the same month is 30 days rather than 31. If a document counts both endpoints as elapsed days, add one to the result.
Calendar days against business days
Calendar days every day counts. Business days skip weekends, and in many definitions skip public holidays too. The gap between the two is not small:
| Span | Calendar days | Business days |
|---|
| Mon 14 Sep – Fri 18 Sep 2026 | 4 | 4 |
| Mon 14 Sep – Fri 25 Sep 2026 | 11 | 9 |
| One calendar year | 365 | ≈ 250 |
A common year has 365 days, which is 52 weeks plus one day, giving about 260 weekdays. Remove the US federal holidays that land on a weekday and the working year is closer to 250 days. That is a difference of roughly 115 days a year, or almost a third of the calendar.
Why a month is not 30 days
Adding "one month" means advancing the calendar month, not adding 30 days. The two produce different answers, and the gap compounds. Take a Net 30 invoice issued on 31 January: thirty days later is 2 March, while one month later is 28 February. Getting this wrong on a recurring invoice shifts the due date every cycle.
When the target month is shorter than the starting day, the result is clamped to that month's last day. 31 January plus one month is 28 February in a common year and 29 February in a leap year. The same rule applies on 31 March, 31 May, 31 August and 30 October. The calculator flags this whenever it happens, so you know the result was adjusted rather than rounded.
Leap years and the calendar
The calculator works on the Gregorian calendar, the same one used for ISO 8601 dates. Leap years add 29 February: 2028 is the next one. Adding one year to 29 February 2024 gives 28 February 2025, because 2025 has no equivalent day. Leap years do not affect ordinary day counts, since 29 February is counted as a normal day.
Three worked examples
1. An invoice with Net 30 terms issued on a Friday
An invoice is dated Friday 18 September 2026 with Net 30 terms. The payment terms run in calendar days, so the due date is Sunday 18 October 2026. Because that lands on a weekend, many businesses treat the effective due date as the following Monday. Switch to the Business days tab and forward 30 working days instead and you land on Friday 30 October 2026, twelve calendar days later. Knowing both numbers matters when a client argues about a late-payment fee.
2. A project quoted at 45 working days
A contractor quotes 45 working days from Monday 21 September 2026. On working days alone that is Friday 20 November 2026. Because the run crosses Thanksgiving, adding the federal holiday exclusion pushes delivery to Monday 23 November 2026. Quoting in working days without accounting for holidays is one of the most common ways a schedule slips.
3. A 90-day return window
A purchase made on 1 June 2026 with a 90-day return window closes on 30 August 2026 in calendar days. If the policy instead says 90 business days, the window runs to Friday 2 October 2026 — more than a month longer. Read the wording carefully: "90 days" and "90 business days" are different policies.
Tying a deadline to the federal calendar? The 2027 federal holiday calendar lists every observed date, including the two that move.
Sources and standards
How we verify this calculator
The date arithmetic follows the Gregorian calendar and ISO 8601 date notation. All calculations are performed on calendar dates with no clock time, so results do not shift with time zone or daylight saving.
Holiday tables and worked examples last verified: 19 September 2026.
FAQ
How many days are between two dates?
Subtract the start date from the end date. September 1 to September 30 is 29 days using the standard convention, which does not count the start day but does count the end day. Add one day if you want both endpoints counted as elapsed days.
Does the date calculator include the start date?
No. The standard count excludes the start day and includes the end day, so September 1 to September 2 is 1 day. This is the convention banks, payroll systems and most contracts use. If both endpoints should count, add one to the result.
How do I add 30 days to a date?
Switch to the Add or subtract tab, enter the start date, set the amount to 30 and the unit to days, then choose Add. Adding 30 calendar days to September 18, 2026 gives October 18, 2026. Remember that 30 days is not the same as one month.
Why does adding one month give a different day of the month?
Because calendar months have different lengths, the result is clamped to the last day when the target month is shorter. January 31 plus one month is February 28 in a common year and February 29 in a leap year. This clamping rule is the standard convention for contract, lease and billing dates.
What is the difference between business days and weekdays?
Business days exclude weekends and, in most definitions, public holidays as well. Weekdays are simply Monday to Friday. In a normal year there are about 260 weekdays but only around 248 to 250 actual business days once the US federal holidays that fall on weekdays are removed.
How many business days are in a year?
A common year has 365 days, which is 52 weeks plus one day, giving 260 weekdays on average. Removing the 11 US federal holidays, of which roughly 8 to 10 fall on a weekday in any given year, leaves about 250 business days.
Does the calculator handle leap years?
Yes. February 29 is treated as a normal calendar day and is counted. A leap year contains 366 days, and 2028 is the next leap year. Adding one year to February 29, 2024 returns February 28, 2025 because 2025 has no 29 February.
Can I count backwards from a deadline?
Yes. Set the end date earlier than the start date and the calculator returns the absolute number of days, or use the Add or subtract tab with the Subtract direction to move a date backwards in days, weeks, months or years.
Are the dates calculated in my local time zone?
All arithmetic runs on calendar dates rather than clock times, so a date is treated as a whole day with no time zone applied. This avoids the classic off-by-one error where a date shifts by a day because of UTC conversion.
Is this calculator suitable for legal or financial deadlines?
It is a general-purpose date calculator and a useful cross-check, but statutory deadlines can follow their own counting rules, such as excluding the trigger day, rolling to the next business day when a deadline lands on a weekend, or using court-specific holiday calendars. Always confirm against the governing document or your jurisdiction's rules.