diff --git a/sphinx/builders/gettext.py b/sphinx/builders/gettext.py
index 1a30dd0fc..1c4789392 100644
--- a/sphinx/builders/gettext.py
+++ b/sphinx/builders/gettext.py
@@ -130,6 +130,8 @@ class I18nBuilder(Builder):
 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)
diff --git a/sphinx/config.py b/sphinx/config.py
index 5ce807f7a..9cbf655f8 100644
--- a/sphinx/config.py
+++ b/sphinx/config.py
@@ -301,7 +301,8 @@ class Config(object):
         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: