python - Unix 时间戳实际上跟踪什么？

Question

我知道 Unix 时间戳定义为自 1970-01-01 00:00:00Z 以来经过的秒数。但是，我找不到给出此定义的明确来源。我还阅读了有关闰秒方面 UTC 和 Unix 时间戳之间关系的各种不同陈述。

This wikipedia page包含所有闰秒的列表。第一个是

1972-06-30 23:59:60

关于Unix时间戳的说法

As for "all modern computer systems", Unix time is completely ignorant about anything except seconds.

来源:HackerNews, brazzy

UNIX time tracks UTC instead of TAI meaning it "corrects" for leap seconds. As a result, UNIX time is not "the number of seconds since epoch" but "86400 * (number of whole days since epoch) + (number of seconds since midnight)", and UNIX time will go forwards (never so far) and backwards on leap seconds (a second will repeat in most implementations as the day goes from 23:59:60 to 00:00:00, as they have the same timestamp).

来源:Hacker News, masklinn

我也读过(但我无法再次找到它 - 在 Stack Overflow 的某处)Unix 时间戳假定每天正好有 24*60*60 秒。海报暗示日子仍然以某种方式保持同步，而闰秒只是“减慢”了真正的秒数。因此，“unix 时间戳秒”可能不是 SI 秒。

可能的答案

我可以看到三个可能的答案:

A1:Unix 时间戳跟踪自 1970-01-01 00:00:00Z 以来的 SI 秒数。这意味着它们与 UTC 相差 27 秒。

A2:Unix 时间戳跟踪“TAI 中经过的秒数”。这意味着将 Unix 时间戳转换为 UTC 的库必须处理闰秒。

A3:Unix 时间戳跟踪“UTC 中经过的秒数”。这意味着在大多数情况下，两个 1 的 Unix 时间戳之间的差异可能是 1 SI 秒，但并非全部。

请为您的答案添加来源。

python

Python datetime 似乎不知道闰秒(？)。

>>> import datetime
>>> a = datetime.datetime(1972, 6, 30, 23, 59, 59)
>>> b = datetime.datetime(1972, 7, 1, 0, 0, 0)
>>> b-a
datetime.timedelta(0, 1)

time 模块似乎将实际闰秒映射到之前的秒数:

>>> import time
>>> t3 = time.mktime((1972, 6, 30, 23, 59, 59, -1, -1, -1))
>>> t4 = time.mktime((1972, 7, 1, 0, 0, 0, -1, -1, -1))
>>> t4 - t3
1.0
>>> t4 = time.mktime((1972, 6, 30, 23, 59, 60, -1, -1, -1))
>>> t4 - t3
1.0

此印象由 issue23574 支持.

Answer 1

A.4.16 Seconds Since the Epoch

Coordinated Universal Time (UTC) includes leap seconds. However, in POSIX time (seconds since the Epoch), leap seconds are ignored (not applied) to provide an easy and compatible method of computing time differences. Broken-down POSIX time is therefore not necessarily UTC, despite its appearance.[...]

Most systems' notion of "time" is that of a continuously increasing value, so this value should increase even during leap seconds. However, not only do most systems not keep track of leap seconds, but most systems are probably not synchronized to any standard time reference. Therefore, it is inappropriate to require that a time represented as seconds since the Epoch precisely represent the number of seconds between the referenced time and the Epoch.

It is sufficient to require that applications be allowed to treat this time as if it represented the number of seconds between the referenced time and the Epoch. It is the responsibility of the vendor of the system, and the administrator of the system, to ensure that this value represents the number of seconds between the referenced time and the Epoch as closely as necessary for the application being run on that system.

来源: http://pubs.opengroup.org/onlinepubs/9699919799/xrat/V4_xbd_chap04.html#tag_21_04_16