Allow using GMT in the HTML last updated format

CHANGES.rst

@@ -48,6 +48,10 @@ Features added
 * #12743: Add :option:`sphinx-build --exception-on-warning`,
   to raise an exception when warnings are emitted during the build.
   Patch by Adam Turner and Jeremy Maitin-Shepard.
+* #12907: Add :confval:`html_last_updated_time_zone` to allow using
+  GMT (universal time) instead of local time for the date-time
+  supplied to :confval:`html_last_updated_fmt`.
+  Patch by Adam Turner.
 
 Bugs fixed
 ----------

doc/conf.py

@@ -53,6 +53,7 @@
 html_use_opensearch = 'https://www.sphinx-doc.org/en/master'
 html_baseurl = 'https://www.sphinx-doc.org/en/master/'
 html_favicon = '_static/favicon.svg'
+html_last_updated_time_zone = 'GMT'
 
 htmlhelp_basename = 'Sphinxdoc'
 

doc/usage/configuration.rst

@@ -1717,6 +1717,14 @@ and also make use of these options.
    The empty string is equivalent to :code-py:`'%b %d, %Y'`
    (or a locale-dependent equivalent).
 
+.. confval:: html_last_updated_time_zone
+   :type: :code-py:`'local' | 'GMT'`
+   :default: :code-py:`'local'`
+
+   Choose GMT (+00:00) or the system's local time zone
+   for the time supplied to :confval:`html_last_updated_fmt`.
+   This is most useful when the format used includes the time.
+
 .. confval:: html_permalinks
    :type: :code-py:`bool`
    :default: :code-py:`True`

sphinx/builders/html/__init__.py

@@ -454,7 +454,10 @@ def prepare_writing(self, docnames: set[str]) -> None:
         last_updated: str | None
         if (lu_fmt := self.config.html_last_updated_fmt) is not None:
             lu_fmt = lu_fmt or _('%b %d, %Y')
-            last_updated = format_date(lu_fmt, language=self.config.language)
+            local_time = self.config.html_last_updated_time_zone == 'local'
+            last_updated = format_date(
+                lu_fmt, language=self.config.language, local_time=local_time
+            )
         else:
             last_updated = None
 
@@ -1323,6 +1326,8 @@ def setup(app: Sphinx) -> ExtensionMetadata:
     app.add_config_value('html_static_path', [], 'html')
     app.add_config_value('html_extra_path', [], 'html')
     app.add_config_value('html_last_updated_fmt', None, 'html', str)
+    app.add_config_value('html_last_updated_time_zone', 'local', 'html',
+                         ENUM('GMT', 'local'))
     app.add_config_value('html_sidebars', {}, 'html')
     app.add_config_value('html_additional_pages', {}, 'html')
     app.add_config_value('html_domain_indices', True, 'html', types={set, list})

sphinx/util/i18n.py

@@ -4,6 +4,7 @@
 
 import os
 import re
+import sys
 from datetime import datetime, timezone
 from os import path
 from typing import TYPE_CHECKING, NamedTuple
@@ -59,6 +60,11 @@ def __call__(  # NoQA: E704
 
     Formatter: TypeAlias = DateFormatter | TimeFormatter | DatetimeFormatter
 
+if sys.version_info[:2] >= (3, 11):
+    from datetime import UTC
+else:
+    UTC = timezone.utc
+
 logger = logging.getLogger(__name__)
 
 
@@ -223,16 +229,26 @@ def babel_format_date(date: datetime, format: str, locale: str,
 
 
 def format_date(
-    format: str, *, date: datetime | None = None, language: str,
+    format: str,
+    *,
+    date: datetime | None = None,
+    language: str,
+    local_time: bool = False,
 ) -> str:
     if date is None:
         # If time is not specified, try to use $SOURCE_DATE_EPOCH variable
         # See https://wiki.debian.org/ReproducibleBuilds/TimestampsProposal
         source_date_epoch = os.getenv('SOURCE_DATE_EPOCH')
         if source_date_epoch is not None:
-            date = datetime.fromtimestamp(float(source_date_epoch), tz=timezone.utc)
+            date = datetime.fromtimestamp(float(source_date_epoch), tz=UTC)
         else:
-            date = datetime.now(tz=timezone.utc).astimezone()
+            date = datetime.now(tz=UTC)
+
+    if local_time:
+        # > If called with tz=None, the system local time zone
+        # > is assumed for the target time zone.
+        # https://docs.python.org/dev/library/datetime.html#datetime.datetime.astimezone
+        date = date.astimezone(tz=None)
 
     result = []
     tokens = date_format_re.split(format)

tests/test_util/test_util_i18n.py

@@ -2,6 +2,7 @@
 
 import datetime
 import os
+import time
 
 import babel
 import pytest
@@ -93,6 +94,24 @@ def test_format_date():
     assert i18n.format_date(format, date=datet, language='en') == '+0000'
 
 
+def test_format_date_timezone():
+    dt = datetime.datetime(2016, 8, 7, 5, 11, 17, 0, tzinfo=datetime.timezone.utc)
+    if time.localtime(dt.timestamp()).tm_gmtoff == 0:
+        raise pytest.skip('Local time zone is GMT')  # NoQA: EM101
+
+    fmt = '%Y-%m-%d %H:%M:%S'
+
+    iso_gmt = dt.isoformat(' ').split('+')[0]
+    fd_gmt = i18n.format_date(fmt, date=dt, language='en', local_time=False)
+    assert fd_gmt == '2016-08-07 05:11:17'
+    assert fd_gmt == iso_gmt
+
+    iso_local = dt.astimezone().isoformat(' ').split('+')[0]
+    fd_local = i18n.format_date(fmt, date=dt, language='en', local_time=True)
+    assert fd_local == iso_local
+    assert fd_local != fd_gmt
+
+
 @pytest.mark.sphinx('html', testroot='root')
 def test_get_filename_for_language(app):
     get_filename = i18n.get_image_filename_for_language