3 ms·

Because unix time isn't a stopwatch that someone started a long time ago. Leap seconds change unix time because unix time is tied to UTC.

by koeng 5y ago

erdos4d 5y ago
I'm not really following. How does the fact that Unix timestamps are in terms of UTC affect whether leap seconds get incorporated into the timestamp or not? Do leap seconds work differently in UTC or something?
- growse 5y ago
  Unix timestamp assumes there are exactly 86400 seconds in a day. Or, in other words, if your timestamp is a multiple of 86400, it's midnight UTC. There are not alway 86400s in 1 day. Midnight is not a whole number of multiples of 86400 after Jan 1st 1970.
  - deleted 5y ago
    [deleted]
  - fivea 5y ago
    > There are not alway 86400s in 1 day. Midnight is not a whole number of multiples of 86400 after Jan 1st 1970. Isn't that an issue with the format you want to convert UNIX time to ? I mean, UNIX time is quite clearly and unambiguously defined as the number of seconds that passed since January 1st 1970. How are UTC's leap seconds any relevant?
    - growse 5y ago
      > I mean, UNIX time is quite clearly and unambiguously defined as the number of seconds that passed since January 1st 1970. It's not though. Unix time is not monotonic. It goes backwards sometimes.
- zokier 5y ago
  Specifically unix timestamp is (number of days since 1970-01-01 × 86400) + (seconds since midnight). The difference arises from the fact that in utc some days are 86401 seconds long. Another way of viewing it is that unix timestamp is seconds since 1970-01-01 minus number of leap seconds that have passed.
- voxic11 5y ago
  > If a leap second is to be inserted, then in most Unix-like systems the OS kernel just steps the time back by 1 second at the beginning of the leap second, so the last second of the UTC day is repeated and thus duplicate timestamps can occur. So certain unix timestamp can represent two different points in time, the last normal second of the day or the leap second that happened after it. This makes (certain) unix timestamps ambiguous.
- masklinn 5y ago
  > Do leap seconds work differently in UTC or something? Leap seconds exist in UTC, that's the entire thing. A leap second getting inserted (which has always been the case so far, though it may eventually change) means a UTC day ends on 23:59:60, which as far as UTC is concerned is perfectly fine, nothing wrong with a minute having 61 seconds. UNIX time though? UNIX time is (86400 * days since epoch) + seconds since midnight. 23:59:60 is 86400, so on the last second of day X you're at 86400 * X + 86400, and on the next second (the first of day X+1) you're at 86400 * (X+1) + 0. Which is the same value. And thus one second repeats.

Because unix time isn't a stopwatch that someone started a long time ago. Leap seconds change unix time because unix time is tied to UTC.

by koeng 5y ago

Because unix time isn't a stopwatch that someone started a long time ago. Leap seconds change unix time because unix time is tied to UTC.

erdos4d 5y ago

I'm not really following. How does the fact that Unix timestamps are in terms of UTC affect whether leap seconds get incorporated into the timestamp or not? Do leap seconds work differently in UTC or something?

growse 5y ago
Unix timestamp assumes there are exactly 86400 seconds in a day. Or, in other words, if your timestamp is a multiple of 86400, it's midnight UTC. There are not alway 86400s in 1 day. Midnight is not a whole number of multiples of 86400 after Jan 1st 1970.
- deleted 5y ago
  [deleted]
- fivea 5y ago
  > There are not alway 86400s in 1 day. Midnight is not a whole number of multiples of 86400 after Jan 1st 1970. Isn't that an issue with the format you want to convert UNIX time to ? I mean, UNIX time is quite clearly and unambiguously defined as the number of seconds that passed since January 1st 1970. How are UTC's leap seconds any relevant?
  - growse 5y ago
    > I mean, UNIX time is quite clearly and unambiguously defined as the number of seconds that passed since January 1st 1970. It's not though. Unix time is not monotonic. It goes backwards sometimes.
zokier 5y ago
Specifically unix timestamp is (number of days since 1970-01-01 × 86400) + (seconds since midnight). The difference arises from the fact that in utc some days are 86401 seconds long. Another way of viewing it is that unix timestamp is seconds since 1970-01-01 minus number of leap seconds that have passed.
voxic11 5y ago
> If a leap second is to be inserted, then in most Unix-like systems the OS kernel just steps the time back by 1 second at the beginning of the leap second, so the last second of the UTC day is repeated and thus duplicate timestamps can occur. So certain unix timestamp can represent two different points in time, the last normal second of the day or the leap second that happened after it. This makes (certain) unix timestamps ambiguous.
masklinn 5y ago
> Do leap seconds work differently in UTC or something? Leap seconds exist in UTC, that's the entire thing. A leap second getting inserted (which has always been the case so far, though it may eventually change) means a UTC day ends on 23:59:60, which as far as UTC is concerned is perfectly fine, nothing wrong with a minute having 61 seconds. UNIX time though? UNIX time is (86400 * days since epoch) + seconds since midnight. 23:59:60 is 86400, so on the last second of day X you're at 86400 * X + 86400, and on the next second (the first of day X+1) you're at 86400 * (X+1) + 0. Which is the same value. And thus one second repeats.