Day Counter

How to use the day counter

Pick a start date and an end date. The counter reports the number of days between them, then breaks the span down into years, months and days, into weeks and days, and into weekdays and weekend days. By default the start date counts and the end date does not, the convention used for durations such as hotel stays or loan interest. If both dates should count, as in a conference that runs from the 3rd through the 5th (three days, not two), turn on Include end day.

Holidays are handled separately because they depend on where you work. Open Settings to tick the holidays you observe, from the US federal list plus Black Friday, Christmas Eve and New Year's Eve, and add any others, such as company days off or regional holidays, one per line. Holidays that fall on a weekday are subtracted from the weekdays to give business days, and the dates are listed under the result so you can check them. Federal holidays that land on a weekend are moved to the observed weekday, the Friday before or the Monday after.

Day counts answer questions such as how long until a deadline, how many working days a project has, or how many days remain in a notice period. If the end date is earlier than the start date, the counter still returns the gap and flags that the dates are reversed.

Counting days from a date

The second form works the other way: it starts from one date and moves a set number of days forward or backward. With count business days only ticked, it steps over Saturdays, Sundays and the holidays you select, which is how many legal, shipping and payment deadlines are written. Fifteen business days from a Monday with no holidays in between is the Monday three weeks later.

Day of the week

Every result shows its weekday, but you can also work out the weekday of any date in your head with a trick devised by the mathematician John Conway, described below.

Doomsdays

In any given year, a set of easy-to-remember dates all fall on the same weekday, which Conway called the year's doomsday. In 2026, for instance, every one of the dates below is a Saturday.

The even months are simple: 4/4, 6/6, 8/8, 10/10 and 12/12. For the odd months from May to November, the mnemonic "working 9 to 5 at the 7-11" gives 5/9 and 9/5, and 7/11 and 11/7. In March, the 14th (Pi Day) works, as does the last day of February, sometimes called March 0. January and February depend on leap years: January 3 and February 28 in common years, January 4 and February 29 in leap years. Other handy doomsdays include Independence Day (July 4), Halloween (October 31) and Boxing Day (December 26).

Anchor day

To find a year's doomsday you start from its century's anchor day. Because the Gregorian calendar repeats exactly every 400 years, the anchors cycle through just four weekdays. For the 1900s the anchor is Wednesday, for the 2000s Tuesday, for the 2100s Sunday and for the 1800s Friday; the pattern then repeats, so the 2200s, like the 1800s, use Friday.

The anchor is simply the doomsday of the century year itself. In 2000, for example, April 4, June 6 and December 12 were all Tuesdays.

The Doomsday rule

With the anchor known, take the last two digits of the year (yy) and work out four numbers: a, how many times 12 goes into yy; b, the remainder; c, how many times 4 goes into b; and d, the sum a + b + c reduced modulo 7. Counting d weekdays forward from the anchor gives the year's doomsday. Finally, step from the nearest doomsday date in the month to your date.

Take July 20, 1969, the day of the first Moon landing. The anchor for the 1900s is Wednesday. For 69: a = 5, b = 9, c = 2, so a + b + c = 16, which leaves 2 after removing multiples of 7. Two days after Wednesday is Friday, so 7/11/1969 was a Friday. July 20 is nine days later, which is one week and two days, so it was a Sunday. The same steps show that 2026's doomsday is Saturday: for 26, a = 2, b = 2 and c = 0, a total of 4, and four days after the Tuesday anchor is Saturday.

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