Stack Overflow asks for your birthday in the format YYYY/MM/DD. Well that's kinda geeky! Only technologists format dates with the year first - I use it because it makes them sortable when in text.

But... what format is that? Why is it used?

If it were ISO 8601 format - it would be YYYYMMDD (no delimiters) or YYYY-MM-DD (hyphens as delimiters).

  • It is the Jeff-Geoff format (pronun: "jefjef"). Standardized by two influential programmers that started a popular Q+A web site. – Uphill Luge Mar 6 '11 at 19:27
  • @Hans: Which is the "Geoff" here? Did you mean Joel? – Jon Skeet Mar 6 '11 at 20:01
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    @Jon: Geoff Dalgas, the Man Behind The Curtain. stackoverflow.com/users/2/geoff-dalgas – Uphill Luge Mar 6 '11 at 20:09
  • @Hans: Ah, right. I see Geoff as become SOVA #00003 in May 2009... was he involved in coding it to start with too, just on a different basis? – Jon Skeet Mar 6 '11 at 20:18
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    @Jon: you're turning my joke into a yoke. – Uphill Luge Mar 6 '11 at 20:22
  • Mandatory XKCD reference: xkcd.com/1179. Also, the date 2013-08-23 can also be written as 2013-235 or 2013235 (see en.wikipedia.org/wiki/ISO_8601#Ordinal_dates). – ErikHeemskerk Aug 23 '13 at 5:11

Just guessing here.

If it's about why there's a slash or an hyphen, I'd say there's no real reason. They had to chose one, and that's the one they're sticking with.

If it's about why the year is first, it's probably because in some country (like France), we put the day first, then month, then the year. Whereas in the USA, they put the month first, then the day, then the year.
And since I never saw YYYY/DD/MM, they probably put the year first to avoid ambiguity in other countries.

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    AFAIK north america is the only place in the world that puts the month first. YYYY-MM-DD (with -, not /) is the technically correct, only un-ambiguous way of asking for a date. – Mark Henderson Mar 6 '11 at 20:46
  • YYYYMMDD, YYYY-DDD and YYYYDDD are also correct, per ISO 8601. – ErikHeemskerk Aug 23 '13 at 5:15

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