DocumentProperties.created and modified are set at import time of the openpyxl module, rather than actual created time

Thank you for openpyxl, it is very useful!

Description

We're creating documents and were surprised to find that the created and modified attributes of openpyxl.packaging.core.DocumentProperties are set to the time that the module is imported, rather than when an individual DocumentProperties instance is created.

Reproducer

from datetime import datetime
import time

before_import = datetime.utcnow()
import openpyxl
after_import_ = datetime.utcnow()

first_props__ = openpyxl.packaging.core.DocumentProperties().created
time.sleep(1.234)
second_props_ = openpyxl.packaging.core.DocumentProperties().created

print(
    f"""\
{before_import = }
{after_import_ = }
{first_props__ = }
{second_props_ = }
{openpyxl.__version__ = }"""
)

Output:

before_import = datetime.datetime(2022, 8, 31, 22, 23, 12, 614431)
after_import_ = datetime.datetime(2022, 8, 31, 22, 23, 12, 798539)
first_props__ = datetime.datetime(2022, 8, 31, 22, 23, 12, 793817)
second_props_ = datetime.datetime(2022, 8, 31, 22, 23, 12, 793817)
openpyxl.__version__ = '3.0.10'

Note that both props timestamps are exactly equal, and occur between the before/after import timestamps.

Proposed patch

One possibility to address would be a change to https://foss.heptapod.net/openpyxl/openpyxl/-/blob/c95449fa6295debe169f265c09182d6d7bd68a33/openpyxl/packaging/core.py#L89-94 like the following, to move the default arg logic (which is evaluated during parsing/on import) to be within the constructor:

                  revision=None,
                  version=None,
-                 created=datetime.datetime.utcnow(),
+                 created=None,
                  creator="openpyxl",
                  description=None,
                  identifier=None,
                  language=None,
-                 modified=datetime.datetime.utcnow(),
+                 modified=None,
                  subject=None,
                  title=None,
                  ):
+        if created is None:
+            created = datetime.datetime.utcnow()
+        if modified is None:
+            modified = datetime.datetime.utcnow()
         self.contentStatus = contentStatus
         self.lastPrinted = lastPrinted

That said, it looks like the behaviour was changed from timestamps-at-creation to timestamps-at-import in 66c7f18a5ab2 and thus this may be intended behaviour?

Work-around

We've been able to work-around this by explicitly setting wb.properties.created = wb.properties.modified = datetime.utcnow() in our code.

---

As I said above, thank you for openpyxl!