Time and date for programmers
Time and date for programmers
Michał Pipa
PHPers Summit, Poznań, 2017-09-02
About me:
- PHP & Symfony
- PHPers
- szkolenia, konsulatcje
phpCE
summit2017
Date format
ISO 8601 was published on 06/05/88 and most recently amended on 12/01/04.
ISO 8601
2017-02-24T23:04:17Z
2017-02-24T23:04:17+00:00
RFC 3339
1985-04-12T23:20:50.52Z
1996-12-19T16:39:57-08:00
RFC 2616
Sun, 06 Nov 1994 08:49:37 GMT
Unix timestamp
number of seconds since midnight, 1 January 1970 (UTC)
Unix timestamp
1488024000
1 500 000 000
Fri Jul 14 04:40:00 CEST 2017
Avoid date formats not specifying time zone
2016-01-10 13:14:15
Programmers are not always aware of ISO 8601. Use it everywhere, UI can be an exception of this rule.
ISO week number
The ISO 8601 definition for week 01 is the week with the Gregorian year's first Thursday in it.
Leap years
Criteria to identify leap year:
- year can be evenly divided by 4
- if year can be evenly divided by 100, it is not a leap year
- unless year can be evenly divided by 400
Leap years
- 1896
- 1904
- ...
- 1996
- 2000
- 2004
next year = year + 365 days
next year = year + 1 year
Easter
Computus
The calculation used to determine the calendar date of Easter
Check if your programming language has function for Easter date
Leap seconds
Universal Time (UT1)
Based on Earth's rotation
Coordinated Universal Time (UTC)
Based on Atomic time
Leap second
- Earth slows down it's rotation
- extra second can be added or removed from UTC
- occurs on midnight UTC
- usually last day of June or December
Leap second insertion
- 23:59:58
- 23:59:59
- 23:59:60
- 00:00:00
Leap second removal
- 23:59:57
- 23:59:58
- 00:00:00
Unix timestamp ignores leap seconds
Leap second insertion
- 1483228798
- 1483228799
- 1483228799
- 1483228800
How to handle leap second? (Linux)
- kernel backward step
- daemon backward step
- ignore leap second and correct by slewing
- client slew
- server slew
Time zones
Time zone naming
- CET
- Europe/Warsaw
Time zone offset is not always full hour
Asia/Kolkata +05:30
Asia/Kathmandu +05:45
Daylight saving time (DST)
Time difference between Warsaw and Sydney
Date: 2017-02-25
Warsaw: UTC + 1
Sydney: UTC + 11
Difference: 10 hours
Date: 2017-03-27
Warsaw: UTC + 2
Sydney: UTC + 11
Difference: 9 hours
Date: 2017-04-03
Warsaw: UTC + 2
Sydney: UTC + 10
Difference: 8 hours
Date: 2017-10-02
Warsaw: UTC + 2
Sydney: UTC + 11
Difference: 9 hours
Date: 2017-10-30
Warsaw: UTC + 1
Sydney: UTC + 11
Difference: 10 hours
DST offset is not always full hour
Australia/Lord_Howe +10:30
Australia/Lord_Howe +11:00 (DST)
Africa/Casablanca suspends DST for about a month (Ramadan)
DST in 2016 for Africa/Casablanca
from 2016-03-27 to 2016-06-05
from 2016-07-10 to 2016-10-30
Time and Date
How to measure elapsed time?
start = now()
f()
end = start - now()
If you want to measure elapsed time use monotonic clock
If your code depends on time make it an input parameter
Instead of this:
function f()
{
$now = time();
// ...
}
Do this:
function f($time)
{
$now = $time;
// ...
}
In your application do not call system clock
interface Clock
{
public function now(): DateTimeImmutable;
}
class SystemClock
{
public function now(): DateTimeImmutable
{
return new DateTimeImmutable();
}
}