strftime fails to zero pad year < 1000 on AWS Lambda

[dsotirho-ucsc]

Issue #2562 added inner entity fields submission_date and update_date as datetime objects, and a null_datetime value to serve as a valid datetime value that can represent None/null while indexed in Elasticsearch.

Originally the value datetime(1, 1, 1, tzinfo=timezone.utc) (converted to the string 0001-01-01T00:00:00.000000Z) was to be used to represent null, however when indexed in sandbox a mapper_parsing_exception occurred.

error.caused_by.reason  Invalid format: "1-01-01T00:00:00.000000Z" is malformed at "-01-01T00:00:00.000000Z"
error.caused_by.type    illegal_argument_exception
error.reason            failed to parse field [contents.files.update_date] of type [date] in document with id '948b7062-15d6-5f13-9b8c-0365f4aac067_42a861e6-2e8a-5470-945a-5e3604d3eb96_2021-05-05T21:24:26.174274Z_exists'
error.root_cause.0.reason   failed to parse field [contents.files.update_date] of type [date] in document with id '948b7062-15d6-5f13-9b8c-0365f4aac067_42a861e6-2e8a-5470-945a-5e3604d3eb96_2021-05-05T21:24:26.174274Z_exists'
error.root_cause.0.type mapper_parsing_exception

The same error was encountered when a 2 and 3 digit year was tested, and the error did not occur when a 4 digit year was used.

As a workaround, the value datetime(1970, 1, 1, tzinfo=timezone.utc) (1970-01-01T00:00:00.000000Z) was used as the None/null value.

Determine why Elasticsearch on sandbox cannot parse the year if it has one or more zeros as a prefix, and why this issue does not occur when testing locally on a developer's machine.

[hannes-ucsc]

Ticket needs reproduction.

[melainalegaspi]

@danielsotirhos spike for reproduction.

[dsotirho-ucsc]

From https://bugs.python.org/issue13305

on linux
>>> datetime(1,  2, 10, 11, 41, 23).strftime("%Y")
'1'

on osx
>>> datetime(1,  2, 10, 11, 41, 23).strftime("%Y")
'0001'

>>> datetime.strptime('0001', '%Y')
datetime.datetime(1, 1, 1, 0, 0)

>>> datetime.strptime('1', '%Y')
ValueError: time data '1' does not match format '%Y'

[dsotirho-ucsc]

Workaround added to #2562 (PR #3026) to enforce format_dcp2_datetime outputs a string with a 4-digit year. Also added doctests to format_dcp2_datetime and parse_dcp2_datetime to make sure they support year '0001'.