diff --git a/src/Membership.sol b/src/Membership.sol
index c94a6eb..5207357 100644
--- a/src/Membership.sol
+++ b/src/Membership.sol
@@ -227,8 +227,12 @@ abstract contract MembershipUpgradeable is Initializable {
 
         if (_sender != membership.holder) revert AttemptedExtensionByNonHolder(_idCommitment);
 
-        // FIXME: see spec: should extension depend on the current block.timestamp?
-        uint256 newGracePeriodStartTimestamp = block.timestamp + uint256(activeStateDuration);
+        // Note: we add the new active period to the end of the ongoing grace period
+        uint256 newGracePeriodStartTimestamp = (
+            membership.gracePeriodStartTimestamp + membership.gracePeriodDuration
+            // FIXME: we must use this membership's activeStateDuration, not global default
+            + uint256(activeStateDuration)
+        );
 
         membership.gracePeriodStartTimestamp = newGracePeriodStartTimestamp;
 
diff --git a/test/WakuRlnV2.t.sol b/test/WakuRlnV2.t.sol
index 1c4e136..e5a9d0f 100644
--- a/test/WakuRlnV2.t.sol
+++ b/test/WakuRlnV2.t.sol
@@ -220,11 +220,16 @@ contract WakuRlnV2Test is Test {
         // Attempt to extend the membership (but now we are the owner)
         vm.expectEmit(true, false, false, false); // only check the first parameter of the event (the idCommitment)
         emit MembershipUpgradeable.MembershipExtended(idCommitment, 0, 0, 0);
+
+        (, uint256 oldGracePeriodStartTimestamp, uint32 oldGracePeriodDuration,,,,) = w.memberships(idCommitment);
         w.extendMemberships(commitmentsToExtend);
+        (, uint256 newGracePeriodStartTimestamp, uint32 newGracePeriodDuration,,,,) = w.memberships(idCommitment);
 
-        (, uint256 newgracePeriodStartTimestamp,,,,,) = w.memberships(idCommitment);
-
-        assertEq(block.timestamp + uint256(w.activeStateDuration()), newgracePeriodStartTimestamp);
+        assertEq(oldGracePeriodDuration, newGracePeriodDuration);
+        assertEq(
+            oldGracePeriodStartTimestamp + oldGracePeriodDuration + uint256(w.activeStateDuration()),
+            newGracePeriodStartTimestamp
+        );
         assertFalse(w.isInGracePeriod(idCommitment));
         assertFalse(w.isExpired(idCommitment));
 
@@ -261,8 +266,8 @@ contract WakuRlnV2Test is Test {
         emit MembershipUpgradeable.MembershipExtended(idCommitment, 0, 0, 0);
         w.extendMemberships(commitmentsToExtend);
 
-        (, uint256 newgracePeriodStartTimestamp, uint32 newGracePeriod,,,,) = w.memberships(idCommitment);
-        uint256 expectedExpirationTimestamp = newgracePeriodStartTimestamp + uint256(newGracePeriod) + 1;
+        (, uint256 newGracePeriodStartTimestamp, uint32 newGracePeriodDuration,,,,) = w.memberships(idCommitment);
+        uint256 expectedExpirationTimestamp = newGracePeriodStartTimestamp + uint256(newGracePeriodDuration) + 1;
         uint256 membershipExpirationTimestamp = w.membershipExpirationTimestamp(idCommitment);
         assertEq(expectedExpirationTimestamp, membershipExpirationTimestamp);
         assertTrue(expectedExpirationTimestamp > ogExpirationTimestamp);