diff --git a/newsfragments/3832.minor b/newsfragments/3832.minor
new file mode 100644
index 000000000..e69de29bb
diff --git a/src/allmydata/storage/immutable.py b/src/allmydata/storage/immutable.py
index 0042673f5..7712e568a 100644
--- a/src/allmydata/storage/immutable.py
+++ b/src/allmydata/storage/immutable.py
@@ -152,11 +152,22 @@ class ShareFile(object):
             self._write_lease_record(f, num_leases, lease_info)
             self._write_num_leases(f, num_leases+1)
 
-    def renew_lease(self, renew_secret, new_expire_time):
+    def renew_lease(self, renew_secret, new_expire_time, allow_backdate=False):
+        # type: (bytes, int, bool) -> None
+        """
+        Update the expiration time on an existing lease.
+
+        :param allow_backdate: If ``True`` then allow the new expiration time
+            to be before the current expiration time.  Otherwise, make no
+            change when this is the case.
+
+        :raise IndexError: If there is no lease matching the given renew
+            secret.
+        """
         for i,lease in enumerate(self.get_leases()):
             if timing_safe_compare(lease.renew_secret, renew_secret):
                 # yup. See if we need to update the owner time.
-                if new_expire_time > lease.get_expiration_time():
+                if allow_backdate or new_expire_time > lease.get_expiration_time():
                     # yes
                     lease = lease.renew(new_expire_time)
                     with open(self.home, 'rb+') as f:
diff --git a/src/allmydata/storage/mutable.py b/src/allmydata/storage/mutable.py
index e6f24679b..de840b89a 100644
--- a/src/allmydata/storage/mutable.py
+++ b/src/allmydata/storage/mutable.py
@@ -298,13 +298,24 @@ class MutableShareFile(object):
             else:
                 self._write_lease_record(f, num_lease_slots, lease_info)
 
-    def renew_lease(self, renew_secret, new_expire_time):
+    def renew_lease(self, renew_secret, new_expire_time, allow_backdate=False):
+        # type: (bytes, int, bool) -> None
+        """
+        Update the expiration time on an existing lease.
+
+        :param allow_backdate: If ``True`` then allow the new expiration time
+            to be before the current expiration time.  Otherwise, make no
+            change when this is the case.
+
+        :raise IndexError: If there is no lease matching the given renew
+            secret.
+        """
         accepting_nodeids = set()
         with open(self.home, 'rb+') as f:
             for (leasenum,lease) in self._enumerate_leases(f):
                 if timing_safe_compare(lease.renew_secret, renew_secret):
                     # yup. See if we need to update the owner time.
-                    if new_expire_time > lease.get_expiration_time():
+                    if allow_backdate or new_expire_time > lease.get_expiration_time():
                         # yes
                         lease = lease.renew(new_expire_time)
                         self._write_lease_record(f, leasenum, lease)
diff --git a/src/allmydata/test/test_storage_web.py b/src/allmydata/test/test_storage_web.py
index e905b240d..38e380223 100644
--- a/src/allmydata/test/test_storage_web.py
+++ b/src/allmydata/test/test_storage_web.py
@@ -485,17 +485,7 @@ class LeaseCrawler(unittest.TestCase, pollmixin.PollMixin):
         return d
 
     def backdate_lease(self, sf, renew_secret, new_expire_time):
-        # ShareFile.renew_lease ignores attempts to back-date a lease (i.e.
-        # "renew" a lease with a new_expire_time that is older than what the
-        # current lease has), so we have to reach inside it.
-        for i,lease in enumerate(sf.get_leases()):
-            if lease.renew_secret == renew_secret:
-                lease = lease.renew(new_expire_time)
-                f = open(sf.home, 'rb+')
-                sf._write_lease_record(f, i, lease)
-                f.close()
-                return
-        raise IndexError("unable to renew non-existent lease")
+        sf.renew_lease(renew_secret, new_expire_time, allow_backdate=True)
 
     def test_expire_age(self):
         basedir = "storage/LeaseCrawler/expire_age"