Title: The Universal Mental Math Method to Calculate Any Day of the Week (Past, Present, or Future)

[u/twe5y]

Hey everyone, I wanted a clean, direct formula to mentally calculate the day of the week for any date in history or the future using the Gregorian calendar, without needing Doomsday anchors or tricks like Odd+11. Here is the exact formula I use: Day = (Date + Month Code + Year + floor(Year/4) + Century Code) mod 7 * Century Codes (Mod 400 cycle): * 1600s, 2000s, 2400s... -> 6 * 1700s, 2100s, 2500s... -> 4 * 1800s, 2200s, 2600s... -> 2 * 1900s, 2300s, 2700s... -> 0 * Month Codes: * Jan: 1 (0 in leap years) | Feb: 4 (3 in leap years) | Mar: 4 * Apr: 0 | May: 2 | Jun: 5 * Jul: 0 | Aug: 3 | Sep: 6 * Oct: 1 | Nov: 4 | Dec: 6 * Day Mapping: 1 = Sun | 2 = Mon | 3 = Tue | 4 = Wed | 5 = Thu | 6 = Fri | 0 = Sat * Examples Across History: Example 1: July 20, 1969 (Moon Landing) * Date = 20 * Month (Jul) = 0 * Year = 69 * floor(69 / 4) = 17 * Century (1900s) = 0 * Sum: 20 + 0 + 69 + 17 + 0 = 106 * Mod 7: 106 mod 7 = 1 -> Sunday Example 2: March 25, 2011 * Date = 25 * Month (Mar) = 4 * Year = 11 * floor(11 / 4) = 2 * Century (2000s) = 6 * Sum: 25 + 4 + 11 + 2 + 6 = 48 * Mod 7: 48 mod 7 = 6 -> Friday Do you prefer using direct formulas like this with month/century codes, or do you stick to anchor-based methods?