diff --git a/misc/tools/postprocess-designspace.py b/misc/tools/postprocess-designspace.py
index 9157a57e4..6f4fc1ec5 100644
--- a/misc/tools/postprocess-designspace.py
+++ b/misc/tools/postprocess-designspace.py
@@ -3,7 +3,7 @@
 from multiprocessing import Pool
 from fontTools.designspaceLib import DesignSpaceDocument
 from ufo2ft.filters import loadFilters
-from datetime import datetime
+from datetime import datetime, timezone
 
 sys.path.append(os.path.abspath(os.path.join(os.path.dirname(__file__), 'tools')))
 from common import getGitHash, getVersion
@@ -15,7 +15,11 @@
 def update_version(ufo):
   version = getVersion()
   buildtag, buildtagErrs = getGitHash()
-  now = datetime.utcnow()
+  sourceDateEpoch = os.environ.get("SOURCE_DATE_EPOCH")
+  if sourceDateEpoch is not None:
+    now = datetime.fromtimestamp(int(sourceDateEpoch), timezone.utc)
+  else:
+    now = datetime.utcnow()
   if buildtag == "" or len(buildtagErrs) > 0:
     buildtag = "src"
     print("warning: getGitHash() failed: %r" % buildtagErrs, file=sys.stderr)