Format issue with strftime and %Y

[bkaznowski]

BPO	44610
Nosy	@iritkatriel, @bkaznowski

^{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 = <Date 2021-07-19.15:38:10.378>
created_at = <Date 2021-07-12.15:23:50.524>
labels = ['type-bug', 'library', '3.9', '3.10']
title = 'Format issue with strftime and %Y'
updated_at = <Date 2021-07-19.15:38:10.377>
user = 'https://github.com/bkaznowski'

bugs.python.org fields:

activity = <Date 2021-07-19.15:38:10.377>
actor = 'bkaznowski'
assignee = 'none'
closed = True
closed_date = <Date 2021-07-19.15:38:10.378>
closer = 'bkaznowski'
components = ['Library (Lib)']
creation = <Date 2021-07-12.15:23:50.524>
creator = 'bkaznowski'
dependencies = []
files = []
hgrepos = []
issue_num = 44610
keywords = []
message_count = 3.0
messages = ['397329', '397707', '397808']
nosy_count = 2.0
nosy_names = ['iritkatriel', 'bkaznowski']
pr_nums = []
priority = 'normal'
resolution = 'duplicate'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue44610'
versions = ['Python 3.9', 'Python 3.10']

[bkaznowski]

When you convert a date pre the year 1000 to a string with %Y as the formatter and then back to a date again then it fails. This is because %Y expects it to be formatted with leading zeroes. For example, the year 1/01/01 (yyyy/mm/dd) should be 0001/01/01 when formatting using %Y/%m/%d. However, %Y returns 1.
You can see this in action here:

from datetime import date, datetime
format = "%Y-%m-%d"
formatted = date.min.strftime("%Y-%m-%d")
datetime.strptime(formatted, format).date()

ValueError: time data '1-01-01' does not match format '%Y-%m-%d' is raised on the forth line.

[iritkatriel]

See also bpo-13305.

[bkaznowski]

Ok, thanks @iritkatriel. I have closed it as a duplicate.

I did notice after posting this that in the cpython source code people work around this issue. Now I can see why.

I guess I will just have to work around this issue too. Luckily it doesn't cause too many problems for what I need.

Thanks!