Sad Sunday: The great JavaScript day-of-week confusion

javascript-day-of-week.js

/**
 * Returns a normalized weekday int from 0..6
 *
 * Sun=0, Mon=1, Tue=2, Wed=3, Thu=4, Fri=5, Sat=6
 *
 * This has caused a number of bugs so an explanation is in
 * order. Javascripts's Date#getDay() methods returns a value from
 * Sun=0...Sat=6. In other words, the week starts on Sunday. However,
 * the ISO 8601 week starts on Monday, so the int values range from
 * Mon=1...Sun=7. The numeric values for Mon...Sat are the same, but
 * Sunday is represented as 0 in Javascript and as 7 in ISO 8601.
 * js-joda is using the ISO 8601 standard.
 *
 * Here we pick Javascript's representation, so we return Sun=0.
 */

export function toJavascriptDayOfWeek(n: number) {
  return n % 7;
}

😂