JavaScript

I'll be very happy to not have to use Date any more. Pop quiz, what's in whatnum?

const vday = new Date('14 February 2025');
const whatnum = vday.getDay() - vday.getMonth();

Err, it's 5... Ha, amazing; that's not even the gotcha I meant to demonstrate. getDay returns the day of the week, the day of the month is returned from getDate.

Take two:

const vday = new Date('14 February 2025');
const whatnum = vday.getDate() - vday.getMonth();

So this is 14 - 2 = 12, right? Nope! The day (from getDate) is 1-based whereas month is 0-based, so 14 - 1 = 13.