FuckWhatDay

Benchmarks

Time taken to find out what day 4 October 2026 is. Lower is better. Measured where it says measured.

FuckWhatDay exe, including starting the process about 10 ms, measured
FuckWhatDay on the boring Python interpreter about 40 ms, measured
Looking at a calendar a second or so, estimated
Asking a colleague varies with the colleague; not measured
Explaining to that colleague why you wrote it in Brainfuck still going

FuckWhatDay is therefore faster than looking at a calendar, provided you were already in a terminal and had already built it.

How it works, for the auditors

The joke is the implementation language. The engineering should not be a joke.

  1. Read eight digits and a line feed, stopping at the first byte it dislikes.
  2. Assemble century, year, month and day, one byte each.
  3. Leap year: the full rule, computed from the century and the year.
  4. Walk the months once, finding the month's length and the days before it.
  5. Check the day fits the month. If not: INVALID, with feeling.
  6. Count from 1 January of year 0 (a Saturday), all arithmetic mod 7.
  7. Print one of seven words. Or the eighth.

Then it is checked against an independent reference on every one of the 73,049 supported dates, every time the tests run. The interpreters it runs on are tested before they are trusted. The source is tested for purity. The tape map is tested cell by cell. A claim is a structure, not a sentence.

Read ARCHITECTURE.md