Time is Hard

A look at why it's such a pain to deal with Date/Time in code, and some interesting corner cases. Given at IPRUG.

Transcript

1. Time is Hard @ChrisSinjo IPRUG – 4th June 2013
2. What’s so interesting?
3. Subtlety
4. Time.now is easy
5. Time.now is easy (We’ll see about this)
6. Most apps care (Conjecture)
7. We won’t cover… Inconsistent servers Unreliable clocks Clocks in space
8. So what can go wrong?
9. Server timezone
10. sinjo@widdershins:~$ irb > Time.now => 2013-05-22 22:10:53 +0100 sinjo@hydrophobe:~$ irb > Time.now => 2013-05-22 21:11:07 +0000
11. sinjo@widdershins:~$ irb > Time.now => 2013-05-22 22:10:53 +0100 sinjo@hydrophobe:~$ TZ=UTC irb > Time.now => 2013-05-22 21:11:07 +0000
12. Time.now is easy
13. Time.now.utc is easier
14. User timezone (browser)
15. var offset = new Date().getTimezoneOffset(); offset = -offset; var time = moment("2010-10-20 14:30 +0000", "YYYY-MM-DD HH:mm Z"); var localTime = time.add("minutes", offset);
16. var offset = new Date().getTimezoneOffset(); offset = -offset; var time = moment("2010-10-20 14:30 +0000", "YYYY-MM-DD HH:mm Z"); var localTime = time.add("minutes", offset); Changes with DST
17. Trust the user’s machine?
18. DST
19. HTTP Logs [27/Oct/2013:01:30:00] "GET /some/page" [27/Oct/2013:01:59:00] "GET /another/page" [27/Oct/2013:01:30:00] "GET /some/page"
20. HTTP Logs [27/Oct/2013:01:30:00 +0100] "GET /some/page" [27/Oct/2013:01:59:00 +0100] "GET /another/page" [27/Oct/2013:01:30:00 +0000] "GET /some/page"
21. HTTP Logs [27/Oct/2013:01:30:00 +0100] "GET /some/page" [27/Oct/2013:01:59:00 +0100] "GET /another/page" [27/Oct/2013:01:30:00 +0000] "GET /some/page" [27/Oct/2013:00:30:00 +0000] "GET /some/page" [27/Oct/2013:00:59:00 +0000] "GET /another/page" [27/Oct/2013:01:30:00 +0000] "GET /some/page"
22. Changes to DST
23. 2011: Russia abandons DST
24. Timezone hopping
25. 2011: Samoa skips a day
26. Offsets are not timezones Tip:
27. Offset: +0000 Zone: "Europe/London"
28. SQL doesn’t get this
29. So how do we get it right?
30. UTC everywhere
31. Localise late
32. Use the libraries
33. Trust IANA data
34. Some fun
35. Days that aren’t
36. 1752: England 11 days gone
37. 1752: England A 9 month year
38. Leap seconds
39. 86,400 seconds in a day
40. 86,400 86,401 seconds in a day
41. 2012: Everywhere The end of the world
42. 2012: Everywhere The end of the world
43. 2012: Everywhere Leap-second kernel bug
44. 2012: Everywhere Leap-second kernel bug
45. 2012: Everywhere but Google Leap-second kernel bug
46. But I digress… (Where were we?)
47. The main things UTC rocks IANA tzdata rocks You need to think
48. Questions? @ChrisSinjo