4 ms·

Something which bugs me about UNIX time is that, contradictory to its colloquial name (epoch time), UNIX time does not increase linearly and does funny stuff on

Something which bugs me about UNIX time is that, contradictory to its colloquial name (epoch time), UNIX time does not increase linearly and does funny stuff on leap seconds.

defrost 1y ago
UNIX time is _an_ example of a type of epoch time, with extra rules and conditions. Not all epoch time counters are UNIX time though. The usual case, when referring to an epoch time counter being used, is a uniform, increasing count of elapsed time in a standard fixed length unit (seconds, cycles, orbits, etc.).
cluckindan 1y ago
Even CLOCK_MONOTONIC doesn’t increase linearly, it is affected by NTP updates. Apparently newer Linux kernels support CLOCK_MONOTONIC_RAW which is not affected by NTP, but even that may not increase linearly: it’s not updated when the system is in standby. Then there is also CLOCK_BOOTTIME which is monotonic and accounts for time spent in standby. Neither of these seem to be POSIX standardised, though.
- grodriguez100 1y ago
  CLOCK_MONOTONIC is affected by NTP skewing but should still always increase monotonically (i.e. never decrease)
  - DaiPlusPlus 1y ago
    What happens if someone makes an honest mistake (or is just malicious) and makes their NTP server run fast?
    - kevindamm 1y ago
      It's system dependent but Linux will generally speed up or slow down the time advancement until the delta from adjtime(...) matches up: https://linux.die.net/man/2/clock_gettime https://linux.die.net/man/2/clock_gettime This clock is not affected by discontinuous jumps in the system time (e.g., if the system administrator manually changes the clock), but is affected by the incremental adjustments performed by adjtime(3) and NTP. https://linux.die.net/man/3/adjtime https://linux.die.net/man/3/adjtime If the adjustment in delta is positive, then the system clock is speeded up by some small percentage (i.e., by adding a small amount of time to the clock value in each second) until the adjustment has been completed. If the adjustment in delta is negative, then the clock is slowed down in a similar fashion.
Joker_vD 1y ago
I am fairly certain POSIX define a day to be exactly 86400 seconds for the matter of calendar conversions/calculations, and deliberately ignores leap seconds: https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/V1_chap04.html#tag_04_19 https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/V1...

Something which bugs me about UNIX time is that, contradictory to its colloquial name (epoch time), UNIX time does not increase linearly and does funny stuff on leap seconds.

defrost 1y ago

UNIX time is _an_ example of a type of epoch time, with extra rules and conditions. Not all epoch time counters are UNIX time though. The usual case, when referring to an epoch time counter being used, is a uniform, increasing count of elapsed time in a standard fixed length unit (seconds, cycles, orbits, etc.).

cluckindan 1y ago

Even CLOCK_MONOTONIC doesn’t increase linearly, it is affected by NTP updates. Apparently newer Linux kernels support CLOCK_MONOTONIC_RAW which is not affected by NTP, but even that may not increase linearly: it’s not updated when the system is in standby. Then there is also CLOCK_BOOTTIME which is monotonic and accounts for time spent in standby. Neither of these seem to be POSIX standardised, though.

grodriguez100 1y ago
CLOCK_MONOTONIC is affected by NTP skewing but should still always increase monotonically (i.e. never decrease)
- DaiPlusPlus 1y ago
  What happens if someone makes an honest mistake (or is just malicious) and makes their NTP server run fast?
  - kevindamm 1y ago
    It's system dependent but Linux will generally speed up or slow down the time advancement until the delta from adjtime(...) matches up: https://linux.die.net/man/2/clock_gettime https://linux.die.net/man/2/clock_gettime This clock is not affected by discontinuous jumps in the system time (e.g., if the system administrator manually changes the clock), but is affected by the incremental adjustments performed by adjtime(3) and NTP. https://linux.die.net/man/3/adjtime https://linux.die.net/man/3/adjtime If the adjustment in delta is positive, then the system clock is speeded up by some small percentage (i.e., by adding a small amount of time to the clock value in each second) until the adjustment has been completed. If the adjustment in delta is negative, then the clock is slowed down in a similar fashion.

Joker_vD 1y ago

I am fairly certain POSIX define a day to be exactly 86400 seconds for the matter of calendar conversions/calculations, and deliberately ignores leap seconds: https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/V1_chap04.html#tag_04_19 https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/V1...