Mind Performance Hacks

Hack #43. Calculate Any Weekday

Quickly calculate the day of the week for any date of the Gregorian calendar—useful for scheduling appointments and meetings!

The imperfect Gregorian calendar, when combined with the imperfect Earth year—which is an even multiple of neither 12 (the number of months) nor 7 (the number of days in the week), but instead an icky 365.24237404 days long (approximately!)—means that, for most of us to find what day of the week a meeting falls on, we have to consult a wall calendar or our PDA.

But what if you had your own perpetual calendar in your head? What if you could, with practice, take just a few seconds to calculate any day of the week from centuries ago and into the distant future, when they finally nudge the Earth into a more reasonable orbit?

You can. Here's how.^1,2

In Action

To calculate any weekday, you basically need to find four numbers, add them together, and then cast out sevens (i.e., calculate that number modulo 7, a simple procedure). In practice, you can do the modulo math as you go along to keep the numbers small and simply keep a running total.

Here are the numbers you need:

• The year-item

• The month-item

• The day-item

• Adjustment

The year-item

Finding the year-item (or key number for the year) is easy. Here's the formula:

(YY + (YY div 4)) mod 7

where YY represents the last two digits of the year.

Modulo Math

The div and mod operators come from integer arithmetic. The div operator is the same as ordinary division, but it discards the remainder. So, ...