Date Calculator
"How many days from 1 January to 10 January?" has two right answers — nine and ten — and which one you want depends on whether you are counting days that pass or dates on a calendar. This calculator gives you both instead of picking for you, takes US federal holidays out of the business-day count, and shows the arithmetic behind every figure.
1. Days between two dates
2. Add or subtract days
Nine days or ten? Both, and the difference costs money
Count from 1 January to 10 January. Nine days pass between the two dates. Ten dates are involved if you count both ends. Neither is wrong, and the one you need depends entirely on what the number is for:
| Situation | Which count | 1 Jan → 10 Jan |
|---|---|---|
| Hotel bill — you pay per night | Days that pass | 9 nights |
| Leave request — you are absent on both days | Counting both ends | 10 days |
| "Within 30 days of receipt" in a contract | Days that pass, usually from the day after | 9 elapsed |
| A ticket valid "for 10 days" | Counting both ends | 10 days of validity |
| Medication — one dose a day | Counting both ends | 10 doses |
The rule that fits almost every case: if you are counting gaps — nights, elapsed time, waiting — you want the smaller number. If you are counting occasions — days present, doses taken, days covered — you want the larger. Most date calculators show one figure and leave you to work out which you were given, which is where the off-by-one in someone's holiday allowance comes from.
Business days, and the holidays actually taken out
"Ten business days" means weekdays, and for anything touching a US bank, court or government office it also means not the eleven federal holidays. This calculator removes them by rule rather than asking you to remember which ones fell in your range, and names the ones it removed so you can check.
| Holiday | Rule |
|---|---|
| New Year's Day | 1 January |
| Birthday of Martin Luther King, Jr. | Third Monday in January |
| Washington's Birthday | Third Monday in February |
| Memorial Day | Last Monday in May |
| Juneteenth National Independence Day | 19 June |
| Independence Day | 4 July |
| Labor Day | First Monday in September |
| Columbus Day | Second Monday in October |
| Veterans Day | 11 November |
| Thanksgiving Day | Fourth Thursday in November |
| Christmas Day | 25 December |
The six with fixed dates move when they land on a weekend: a Saturday holiday is observed the Friday before, a Sunday holiday the Monday after. That is why 4 July 2026, a Saturday, is a day off on Friday 3 July — and why New Year's Day can pull a holiday back into the previous December.
What this does not know: state holidays, your employer's shutdown days, and every holiday outside the United States. If your deadline is set by a company or a court in another country, treat the federal-holiday figure as a starting point and subtract the rest yourself.
Handy rules of thumb
- A year is 365 days (366 in leap years — years divisible by 4, except century years not divisible by 400).
- 30 days ≈ 4 weeks and 2 days; 90 days ≈ just under 3 months.
- 52 weeks is 364 days, which is why your birthday shifts one weekday per year (two after a leap day).
- Ten business days is two weeks of work, but almost never fourteen calendar days once a holiday lands in the middle.
Why two date calculators can give you different answers
Date arithmetic looks like it should have one right answer, and for "days between two dates" it very nearly does. For anything involving months it does not, and the disagreements are not bugs — they are different defensible choices. Knowing which choice a tool made is the difference between trusting a deadline and being caught out by it.
Adding a month to the 31st
What is 31 January plus one month? There is no 31 February, so something has to give, and there are two common answers:
| Rule | 31 Jan + 1 month | Used by |
|---|---|---|
| Clamp to the last valid day | 28 Feb (29 in a leap year) | This tool, most spreadsheet functions, most contracts |
| Overflow into the next month | 3 March | Some programming languages' naive date libraries |
Clamping is the convention almost everywhere it matters, because it preserves the useful property that adding a month never skips a month. But it has a consequence worth knowing: month arithmetic is not reversible. 31 January plus one month is 28 February, and 28 February minus one month is 28 January — you do not get back to where you started. If a schedule matters, count in days rather than months, or anchor every date to the same starting point rather than stepping forward repeatedly.
Leap years, and the century rule people forget
A year is a leap year if it divides by 4 — except century years, which must also divide by 400. So 2000 was a leap year and 1900 was not, which is why date arithmetic written in the 1990s so often broke on spans crossing 1900. The next exception is 2100. Any calculation involving 29 February also needs a rule for what "one year later" means: this tool treats 29 February plus one year as 28 February, the same clamping rule as above.
Time zones and daylight saving
This calculator counts calendar days, not elapsed hours, which is what you almost always want and is the reason it does not ask for a time zone. The two are genuinely different: the day a region moves onto summer time is 23 hours long and the day it moves off is 25. A tool that counts elapsed hours and divides by 24 will be off by a fraction of a day across such a boundary and can round to a different answer. Counting calendar days sidesteps the problem entirely — 1 March to 1 April is 31 days no matter what the clocks did in between.
Frequently asked questions
Does the calculator handle leap years?
Yes — it uses the real calendar, so February 29, month lengths, and century rules are all handled exactly.
What does "90 days from today" mean for a deadline?
Usually the 90th day counting the day after today as day 1. Enter today in the second calculator and add 90; if your contract counts today as day 1, add 89 instead.
How do I count months between dates?
The result shows a years/months/days breakdown alongside total days. Months vary in length, so "3 months" is defined calendar-to-calendar (Jan 15 to Apr 15).
Is January 1 to January 10 nine days or ten?
Both, depending on what you are counting. Nine days pass between them, which is what a deadline or a hotel bill means. Ten dates are involved if you count both ends, which is what a leave request or a ticket valid "for 10 days" means. The calculator shows both figures rather than making you pick in advance.
Which holidays are subtracted from the business-day count?
The eleven US federal holidays set by 5 U.S.C. 6103, using the observed date — a holiday on a Saturday is observed the Friday before, and one on a Sunday the Monday after. State holidays, company shutdowns and holidays outside the US are not included, and the calculator names the ones it removed so you can check them.
What date is 10 business days from today?
Set the second calculator to "working days" and enter 10. It counts forward over weekdays only, skipping federal holidays, which is what shipping and legal deadlines normally mean by business days.
Sources
- 5 U.S.C. § 6103 — Holidays: the statute that names the eleven federal holidays and sets the in-lieu-of rule for those falling at a weekend.
- US Office of Personnel Management — Federal Holidays: the published observed dates each year, which the rules in this calculator reproduce.