Day Counter Calculator

How to Use the Day Counter

1. Select Dates: Use the drop-down menus to choose a starting month, date, and year.
2. Include End Day: Check the “include end day” box if you want the final day counted. For example, to count from March 29th to a project deadline on April 24th, you can either:
   • Select March 29th to April 24th and check “include end day,” or
   • Select March 29th to April 25th without checking the box.
3. Settings: Click the “Settings” link to customize options like including US holidays or adding custom holidays.

The calculator assumes a Monday-to-Friday workweek, with weekends on Saturday and Sunday. It breaks results into weekdays and weekend days and includes holidays if selected.

Counting Days from a Date

Enter a start date and the number of days to add or subtract. Check the “count business days only” box to exclude weekends. For instance, adding 7 business days from a Monday will land on the next Wednesday (skipping the weekend).

Day of the Week Calculation

The calculator also reveals the day of the week for your selected dates. One common method for this is John Conway’s Doomsday Rule — an algorithm that can be done mentally with practice.

Doomsday Rule Overview

The Doomsday Rule relies on “doomsdays” — specific dates that always fall on the same weekday within a year. Each year has a different doomsday, but the dates stay the same. Here’s a quick reference:

Doomsdays by Month:

*Leap years shift January’s doomsday to 1/4 and February’s to 2/29.

For even months (except February), the doomsday falls on the matching date (e.g., 4/4, 6/6). Odd months have memory tricks: “I work from 9 to 5 at 7-11” — 9/5, 7/11, and their reversals 5/9, 11/7.

Anchor Day by Century

Each century has an “anchor day” — the weekday all doomsdays revolve around:

The Gregorian calendar repeats every 400 years, so the anchor day for 1500 is the same as 1900 (Wednesday), and for 3000, it matches 2200 (Friday).

Doomsday Rule Steps

1. Find the Century’s Anchor Day:
   • e.g., 2200 = Friday.
2. Divide the Last Two Year Digits by 12:
   • 92 ÷ 12 = 7 (remainder 8). So, b = 7.
3. The Remainder Becomes c:
   • c = 8.
4. Divide c by 4:
   • 8 ÷ 4 = 2 (d = 2).
5. Add It All Up:
   • e = 5 (anchor) + 7 + 8 + 2 = 22.
6. Simplify the Sum:
   • Subtract 7 repeatedly until the result is ≤ 6: 22 – 7(3) = 1 (Monday).
7. Identify the Nearest Doomsday:
   • For 3/15, the closest doomsday is 3/14.
8. Count Forward or Back:
   • 3/15 is one day after Monday (3/14), so it’s Tuesday.

✅ Result: 3/15/2292 was a Tuesday.

This algorithm, with practice, becomes quick and intuitive — perfect for impressing friends or solving date puzzles on the fly!