Add pointers to the SOURCE_DATE_EPOCH specification.

Author: Alexis Bienvenüe

sphinx/builders/gettext.py

@@ -130,6 +130,8 @@ def write_doc(self, docname, doctree):
 timestamp = time()
 tzdelta = datetime.fromtimestamp(timestamp) - \
     datetime.utcfromtimestamp(timestamp)
+# set timestamp from SOURCE_DATE_EPOCH if set
+# see https://reproducible-builds.org/specs/source-date-epoch/
 source_date_epoch = getenv('SOURCE_DATE_EPOCH')
 if source_date_epoch is not None:
     timestamp = float(source_date_epoch)

sphinx/config.py

@@ -301,7 +301,8 @@ def __init__(self, dirname, filename, overrides, tags):
         self.extensions = config.get('extensions', [])
 
         # correct values of copyright year that are not coherent with
-        # the SOURCE_DATE_EPOCH environment variable:
+        # the SOURCE_DATE_EPOCH environment variable (if set)
+        # See https://reproducible-builds.org/specs/source-date-epoch/
         if getenv('SOURCE_DATE_EPOCH') is not None:
             for k in ('copyright', 'epub_copyright'):
                 if k in config: