Day Counter

This tool calculates how many days separate two dates, counting workdays, weekends and holidays. Press the “Settings” link to set up holiday dates.

Count Days from a Date

The day counter or days calculator above can be used in situations such as counting down to a birthday, counting the number of days into a pregnancy, the number of business days left for a project, etc.

How to use the day counter

Pick the start month, day and year from the drop‑down lists to begin counting. If you want the final day counted, tick the “include end day” option. For instance, a deadline at 23:59 on 24 April and today being 29 March – choose those dates and enable the box to count the 24th. Leaving the box unchecked and setting the end date to 25 April yields an identical total.

More options are available under the “Settings” link. Decide if holidays should be taken into account. When you opt in, pick the standard US holidays you want and/or add custom dates in the table beneath. The output will also show how many holidays fall within the selected interval.

The tool computes the interval between two dates based on a standard Monday‑Friday workweek with Saturday and Sunday as weekends. By default it splits the total into weekday and weekend days, and you can additionally factor in public holidays—both counting them and displaying their names. The start day is always counted as a whole day; the end day is counted only when the “include end day” box is checked, otherwise it is excluded.

Counting days from a date

Enter a start date and a number of days to add or subtract, and the program returns the resulting date. The “count business days only” option tells the engine to ignore weekends when adding days. So, starting on a Monday and adding 7 days with the box ticked will land on the following Wednesday, because Saturday and Sunday are omitted from the count.

Day of the week

It also displays the weekday for any date you pick. Numerous methods exist to compute weekdays; one popular technique is Conway’s “Doomsday” rule, which can be performed mentally after a bit of practice.

Doomsdays

The method relies on “doomsdays,” particular calendar dates that share the same weekday within a given year. While the dates themselves stay constant, the weekday they correspond to shifts annually. The following table lists the doomsday for each month.

Memorising these anchor dates helps you determine the weekday of any date. For the even months (except February) the doomsday falls on the day that matches the month number (4/4, 6/6, 8/8, 10/10, 12/12). February’s doomsday is the month’s final day – the 28th in common years, the 29th in leap years. January’s is on 1/3, or 1/4 in a leap year. March’s can be recalled by the first three digits of π (3.14 → 3/14). The remaining odd months are easier with the rhyme “I work from 9 to 5 at 7‑11.” July’s doomsday is 7/11, and November’s is the same numbers swapped (11/7). September’s is remembered as “working 9 to 5” → 9/5, while May’s matches the flipped version 5/9.

Anchor day

An anchor day is the doomsday that applies to an entire century. The yearly doomsday is derived from that century’s anchor. For instance, the year 2000’s doomsday fell on Tuesday, 1900’s on Wednesday, and 2100’s will be Sunday. Because the Gregorian calendar repeats every 400 years, you only need to know the anchor days for four consecutive centuries. Any other century’s anchor can be worked out by comparing it with a set of four known anchors, such as those for 1900, 2000, 2100 and 2200, listed below.

Consequently, the anchor for 1500 (1900 minus 400) is also Wednesday. Adding two 400‑year cycles to 2200 gives 3000, whose anchor day will be Friday, and the pattern continues.

To perform the algorithm, it is also necessary to assign numbers to each day of the week:

Given that the above is memorized (or can be referenced), determining the day of the week given any date just requires the use of basic arithmetic and the following set of rules.

The Doomsday rule

The algorithm is as follows:

1. Determine the anchor day for the given century; assign this to the variable a.
2. Divide the last two digits in the year by 12; assign the result, ignoring any remainder, to b. If the last two digits in the year is less than 12, b = 0.
3. Assign any remainder to c. If there is no remainder, c is 0.
5. Take c, divide it by 4 and discard any leftover fraction; if c is under 4, treat it as zero.
6. Find the sum of a + b + c + d; assign the result to e.
7. Subtract 7 from e until e is 6 or less (this simplifies the arithmetic since any day +/- some multiple of 7 will be the same day); assign the result to f. f represents the day on which doomsday falls in the given year.
8. Determine the closest doomsday to the selected date (pay attention to whether the year is a leap year if the chosen date is in January or February). For example, if the selected date is 4/17, the closest doomsday is 4/4.
9. Count forward or back from the closest doomsday to the selected date, keeping in mind that every +/- 7 days will be the same day, so 4/11, 4/18, 4/25 occur on the same day as 4/4. Each of these dates can be thought of as doomsdays, so if the selected date is 4/15, that is 4 days after a doomsday equivalent, 4/11, or 3 days before another doomsday equivalent, 4/18. If doomsday that year is Thursday, adding 3 days, or subtracting four days, would give the same result: Sunday.

Example:

What day was 3/15/2292?

1. For the year 2200 the century’s anchor lands on Friday, so a = 5.
2. 92 ÷ 12 equals 7 with a remainder of 8, therefore b = 7.
3. The remainder, 8, becomes the value of c.
4. Dividing 8 by 4 yields 2, thus d = 2.
5. Adding the pieces together gives e = 5 + 7 + 8 + 2 = 22.
6. Subtracting 3×7 from 22 leaves 1, which corresponds to f = 1, i.e., Monday.
7. The nearest doomsday before 3/15 is 3/14.
8. Because 3/15 is one day after that Monday doomsday, 1 + 1 = 2, which is Tuesday.

Thus, 3/15/2292 was a Tuesday.