diff --git a/ChangeLog b/ChangeLog
index 59c06cb..34df70d 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -16,6 +16,8 @@ UNRELEASED Version 0.7.0
       correct when the optional k= tag is not present in the DKIM public key
       record
     - Fix -v verbose reporting in dkimverify.py
+    - Fix unbound local variable error when processing signatures with an x
+      tag, but no t tag (LP: #1739637)
 
 2017-05-30 Version 0.6.2
     - Fixed problem with header folding that caused the first line to be
diff --git a/dkim/__init__.py b/dkim/__init__.py
index aa3cfbe..fc4d369 100644
--- a/dkim/__init__.py
+++ b/dkim/__init__.py
@@ -260,6 +260,8 @@ def validate_signature_fields(sig, mandatory_fields=[b'v', b'a', b'b', b'bh', b'
             raise ValidationError(
               "x= value is not a decimal integer (%s)" % sig[b'x'])
         x_sign = int(sig[b'x'])
+        now = int(time.time())
+        slop = 36000 # 10H leeway for mailers with inaccurate clocks
         if x_sign < now - slop:
             raise ValidationError(
                 "x= value is past (%s)" % sig[b'x'])