Definitions of time

[RobertBilling]

BPO	36240
Nosy	@mdickinson, @abalkin, @pganssle

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2019-03-08.14:28:59.094>
labels = ['type-feature', '3.7', 'docs']
title = 'Definitions of time'
updated_at = <Date 2019-03-11.18:51:12.911>
user = 'https://bugs.python.org/RobertBilling'

bugs.python.org fields:

activity = <Date 2019-03-11.18:51:12.911>
actor = 'p-ganssle'
assignee = 'docs@python'
closed = False
closed_date = None
closer = None
components = ['Documentation']
creation = <Date 2019-03-08.14:28:59.094>
creator = 'Robert Billing'
dependencies = []
files = []
hgrepos = []
issue_num = 36240
keywords = []
message_count = 3.0
messages = ['337482', '337675', '337691']
nosy_count = 5.0
nosy_names = ['mark.dickinson', 'belopolsky', 'docs@python', 'p-ganssle', 'Robert Billing']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue36240'
versions = ['Python 3.7']

Linked PRs

• gh-80421: Correct definitions of time #130984
• [3.13] gh-80421: Correct definitions of time (GH-130984) #131022
• [3.12] gh-80421: Correct definitions of time (GH-130984) #131023

[RobertBilling]

https://docs.python.org/3.7/library/time.html contains the text "UTC is Coordinated Universal Time (formerly known as Greenwich Mean Time, or GMT)". This is not strictly true. Referring to https://en.wikipedia.org/wiki/Coordinated_Universal_Time the definition of UTC is in terms of frequency standards, GMT in terms of astronomy. Hence with GMT each minute has exactly 60 seconds, but the length of the second may vary slightly to account for changes in the Earth's rotation. With UTC each second is the same length, but "leap seconds" can be inserted or removed giving 59 and 61 second minutes. The leap seconds keep the two systems in sync to less than one second.

This of course only matters for the most critical applications, but it would be worth documenting correctly.

[abalkin]

How about replacing "formerly known as Greenwich Mean Time, or GMT" with "which superseded Greenwich Mean Time or GMT as the basis of international timekeeping"?

I don't think Python reference manual is the right place to explain the difference between UTC and GMT, but since we have time.gmtime() function, GMT should still be mentioned.

[pganssle]

I also think it would be ideal to avoid getting into too much detail about the definitions of UTC and GMT in a general sense. Instead, we should probably refer to some better source on the matter and maybe focus on how UTC and GMT are used *in this document*?

For example, the gmtime function is explicitly defined in terms of UTC in the documentation, and just has a possibly slightly inaccurate name. Perhaps a wording like "Occasionally the abbreviations 'GMT' and 'UTC' are used interchangeably, despite the fact that this is somewhat inaccurate. For more information about the difference between UTC and GMT, see ."

As something of an aside, the same bullet point says this:

The acronym UTC is not a mistake but a compromise between English and French.

This came up recently on the tz mailing list, where it was claimed that there is no contemporary evidence to support this: https://mm.icann.org/pipermail/tz/2019-March/027736.html

It may be worth removing this sentence or rewording it to be more neutral, like "The acronym UTC is not a mistake but conforms to an earlier, language-agnostic naming scheme for time standards: UT0, UT1, etc." I can move the discussion of the "UTC acronym" wording into a separate ticket if it's distracting from this one.