Day Counter

Two calculators sit above this. The first counts the days between two dates and splits the total into weekend days, holidays and business days, drawing the span on a calendar. The second takes a date and a number of days and tells you where you land, optionally counting only working days.

How to use the day counter

Pick a start month, day and year, then the same for the end. The start date always counts as a full day. The end date does not, unless you tick include end day.

That checkbox is the part worth understanding, because it is where most off-by-one errors come from. If something is due at 11:59 PM on 24 April and today is 29 March, you can either select those two dates and tick the box, or select 25 April and leave it unticked. Both give the same answer, because the box adds exactly one day.

The Settings button opens holiday options. Choose whether holidays are counted at all, tick which of the common US holidays apply, and add your own by name and date for anything not listed, such as a company shutdown or a regional holiday. The calculator then reports how many fell inside your span and names them.

The result assumes a Monday to Friday working week with Saturday and Sunday as the weekend. Where a holiday lands on a weekend it is counted once, as a weekend day, so weekend days plus holidays plus business days always add back to the total rather than double-counting.

Counting days from a date

The second calculator takes a start date and a number of days to add or subtract. The count business days only box changes what a day means. Left unticked, seven days after a Monday is the following Monday. Ticked, weekends are skipped entirely, so seven business days after a Monday is the Wednesday of the following week.

This is how contractual and legal deadlines usually work, and mixing the two conventions is a common way to miss one. A contract giving "10 days to respond" and one giving "10 business days" differ by four calendar days, and across a holiday period the gap grows further.

Why date arithmetic is awkward

Days are the only unit here that never varies. Months run from 28 to 31, years from 365 to 366, and the leap rule itself has an exception to its exception: a year divisible by 4 is a leap year, except centuries, except centuries divisible by 400. So 2000 was a leap year and 1900 was not.

Adding "one month" to 31 January has no single correct answer, which is why this calculator works in days throughout and leaves months out of the arithmetic. Counting days is unambiguous; counting months is a convention.

Working out the day of the week

The calculator names the weekday for every date it shows. Several algorithms produce it, and the most approachable is the Doomsday rule, devised by the mathematician John Conway, which can be done in your head with practice.

It rests on the observation that certain dates always fall on the same weekday within any given year. That shared weekday is that year's doomsday, and the dates are called doomsdays.

Doomsdays by month

Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
1/3*	2/28*	3/14	4/4	5/9	6/6	7/11	8/8	9/5	10/10	11/7	12/12

* In a leap year January's doomsday moves to 1/4 and February's to 2/29.

The even months are the easy half: 4/4, 6/6, 8/8, 10/10 and 12/12 are all doomsdays. February's is the last day of the month, the 28th normally and the 29th in a leap year, and January's is the 3rd, or the 4th in a leap year. March is the 14th, memorable as 3/14, the first three digits of pi.

The remaining odd months follow the mnemonic "I work from 9 to 5 at 7-11". That gives 9/5 and 5/9 as a swapped pair, and 7/11 and 11/7 as another. Learn one of each pair and the other comes free.

Anchor days by century

Each century has an anchor day, which is the doomsday for its year 00. The Gregorian calendar repeats every 400 years, so only four need memorising:

1900	2000	2100	2200
Wednesday	Tuesday	Sunday	Friday

Anything outside that span comes from adding or subtracting 400 years, so 1500 shares Wednesday with 1900, and 3000 will share Friday with 2200.

Numbering the days

Sunday	Monday	Tuesday	Wednesday	Thursday	Friday	Saturday
0	1	2	3	4	5	6

The Doomsday rule step by step

1. Take the anchor day for the century and call it a.
2. Divide the last two digits of the year by 12, ignoring any remainder, and call that b.
3. Call the remainder from that division c.
4. Divide c by 4, ignoring the remainder, and call it d.
5. Add them: e = a + b + c + d.
6. Subtract 7 repeatedly until the result is 6 or less. That is f, the weekday number of that year's doomsday.
7. Find the nearest doomsday to your date, watching for a leap year if the date is in January or February.
8. Count forward or back from it. Every 7 days lands on the same weekday, so 4/11, 4/18 and 4/25 all share 4/4's weekday.

Worked through on 15 March 2292: the anchor for 2200 is Friday, so a = 5. Ninety-two divided by 12 is 7 remainder 8, so b = 7 and c = 8. Eight divided by 4 is 2, so d = 2. Adding gives e = 22, and subtracting 21 leaves f = 1, which is Monday. March's doomsday is 3/14, so 3/14/2292 was a Monday and 3/15 was a Tuesday.

A quicker check on the current year: 2026 has a = 2 for the 2000s, b = 2, c = 2, d = 0, so e = 6 and doomsday is Saturday. That makes 8/8/2026 a Saturday, and the calculator agrees.

What people count days for

Countdowns are the obvious use: days to a birthday, a wedding, an exam or a launch. Pregnancy is tracked the same way, since gestational age is counted in days from the last period and then reported in weeks.

The stricter uses are where the end-day question starts to matter. Notice periods, statutory deadlines and payment terms are all defined as a number of days from a specified date, and whether that date itself counts is written into the rule rather than left to common sense. Court filing deadlines are frequently counted in business days and often exclude public holidays as well, which is exactly the split the first calculator reports.

Travel limits work on rolling day counts too. The Schengen area allows 90 days of stay within any rolling 180-day window, so the question is not how long this trip is but how many days you have already used inside the window ending on your intended departure. Counting that by hand across several trips is where mistakes happen.

Project planning uses the business day figure directly. A task quoted as fifteen working days is three calendar weeks only if no holiday falls inside it, and across late December that same fifteen days can stretch to nearly four weeks.

Reading your result

Check the include end day box before anything else, since it is the difference between 11 days and 12 for the same pair of dates. Ask whether the last day is one you are counting or a deadline you are counting up to.

If the business day figure looks wrong, check the holiday settings. With holidays switched off, only weekends are removed, and a span covering Thanksgiving or Christmas will read as more working days than the office actually has.

Everything here uses a Monday to Friday week. If your working week differs, take the weekend figure as Saturdays and Sundays rather than as days off, and adjust from there. To add or subtract hours and minutes rather than whole days, use the Time Calculator, and for the gap between two times on the same day use the Hours Calculator.