diff --git a/src/TwabController.sol b/src/TwabController.sol
index d6fc375..b16b211 100644
--- a/src/TwabController.sol
+++ b/src/TwabController.sol
@@ -179,7 +179,7 @@ contract TwabController {
    * @return True if the TwabController is shutdown at the given timestamp, false otherwise.
    */
   function isShutdownAt(uint256 timestamp) external view returns (bool) {
-    return TwabLib.isShutdownAt(timestamp, PERIOD_OFFSET);
+    return TwabLib.isShutdownAt(timestamp, PERIOD_LENGTH, PERIOD_OFFSET);
   }
 
   /**
@@ -187,7 +187,7 @@ contract TwabController {
    * @return The largest timestamp at which the TwabController can record a new observation.
    */
   function lastObservationAt() external view returns (uint256) {
-    return TwabLib.lastObservationAt(PERIOD_OFFSET);
+    return TwabLib.lastObservationAt(PERIOD_LENGTH, PERIOD_OFFSET);
   }
 
   /**
diff --git a/src/libraries/TwabLib.sol b/src/libraries/TwabLib.sol
index 2193c28..5f074dd 100644
--- a/src/libraries/TwabLib.sol
+++ b/src/libraries/TwabLib.sol
@@ -109,7 +109,7 @@ library TwabLib {
     // record a new observation if the delegateAmount is non-zero and time has not overflowed.
     isObservationRecorded =
       _delegateAmount != uint96(0) &&
-      block.timestamp <= lastObservationAt(PERIOD_OFFSET);
+      block.timestamp <= lastObservationAt(PERIOD_LENGTH, PERIOD_OFFSET);
 
     accountDetails.balance += _amount;
     accountDetails.delegateBalance += _delegateAmount;
@@ -172,7 +172,7 @@ library TwabLib {
     // record a new observation if the delegateAmount is non-zero and time has not overflowed.
     isObservationRecorded =
       _delegateAmount != uint96(0) &&
-      block.timestamp <= lastObservationAt(PERIOD_OFFSET);
+      block.timestamp <= lastObservationAt(PERIOD_LENGTH, PERIOD_OFFSET);
 
     unchecked {
       accountDetails.balance -= _amount;
@@ -251,7 +251,7 @@ library TwabLib {
       return 0;
     }
     // if this is for an overflowed time period, return 0
-    if (isShutdownAt(_targetTime, PERIOD_OFFSET)) {
+    if (isShutdownAt(_targetTime, PERIOD_LENGTH, PERIOD_OFFSET)) {
       return 0;
     }
     ObservationLib.Observation memory prevOrAtObservation = _getPreviousOrAtObservation(
@@ -271,9 +271,10 @@ library TwabLib {
    */
   function isShutdownAt(
     uint256 timestamp,
+    uint32 PERIOD_LENGTH,
     uint32 PERIOD_OFFSET
   ) internal pure returns (bool) {
-    return timestamp > lastObservationAt(PERIOD_OFFSET);
+    return timestamp > lastObservationAt(PERIOD_LENGTH, PERIOD_OFFSET);
   }
 
   /**
@@ -282,9 +283,10 @@ library TwabLib {
    * @return The largest timestamp at which the TwabController can record a new observation.
    */
   function lastObservationAt(
+    uint32 PERIOD_LENGTH,
     uint32 PERIOD_OFFSET
   ) internal pure returns (uint256) {
-    return uint256(PERIOD_OFFSET) + type(uint32).max;
+    return uint256(PERIOD_OFFSET) + (type(uint32).max / PERIOD_LENGTH) * PERIOD_LENGTH;
   }
 
   /**
@@ -311,7 +313,7 @@ library TwabLib {
     }
 
     // if the range extends into the shutdown period, return 0
-    if (isShutdownAt(_endTime, PERIOD_OFFSET)) {
+    if (isShutdownAt(_endTime, PERIOD_LENGTH, PERIOD_OFFSET)) {
       return 0;
     }
 
diff --git a/test/TwabController.t.sol b/test/TwabController.t.sol
index 1ea19dd..8c2caed 100644
--- a/test/TwabController.t.sol
+++ b/test/TwabController.t.sol
@@ -148,12 +148,13 @@ contract TwabControllerTest is BaseTest {
   function testIsShutdownAt() public {
     assertEq(twabController.isShutdownAt(PERIOD_OFFSET), false, "at beginning");
     assertEq(twabController.isShutdownAt(PERIOD_OFFSET + PERIOD_LENGTH), false, "after first period");
-    assertEq(twabController.isShutdownAt(type(uint32).max + uint256(PERIOD_OFFSET)), false, "at end");
+    assertEq(twabController.isShutdownAt((type(uint32).max/PERIOD_LENGTH)*PERIOD_LENGTH + uint256(PERIOD_OFFSET)), false, "at end of last period");
+    assertEq(twabController.isShutdownAt(type(uint32).max + uint256(PERIOD_OFFSET)), true, "at end");
     assertEq(twabController.isShutdownAt(type(uint32).max + uint256(PERIOD_OFFSET) + 1), true, "after end");
   }
 
   function testLastObservationAt() public {
-    assertEq(twabController.lastObservationAt(), uint256(PERIOD_OFFSET) + type(uint32).max);
+    assertEq(twabController.lastObservationAt(), uint256(PERIOD_OFFSET) + (type(uint32).max/PERIOD_LENGTH)*PERIOD_LENGTH);
   }
 
   function testGetBalanceAt_beforeHistoryStarted() public {
@@ -948,6 +949,18 @@ contract TwabControllerTest is BaseTest {
     assertEq(twabController.delegateOf(mockVault, alice), bob);
   }
 
+  function testMint_lastObservation() public {
+    vm.startPrank(mockVault);
+    uint96 _amount = 1000e18;
+    uint lastAt = twabController.lastObservationAt();
+    console2.log("LAST AT", lastAt);
+    vm.warp(lastAt);
+    twabController.mint(alice, _amount);
+    vm.warp(lastAt + PERIOD_LENGTH);
+    assertEq(twabController.getBalanceAt(mockVault, alice, lastAt), _amount);
+    vm.stopPrank();
+  }
+
   function testMint_toZero() public {
     vm.startPrank(mockVault);
 
diff --git a/test/TwabLib.t.sol b/test/TwabLib.t.sol
index f1de561..386b947 100644
--- a/test/TwabLib.t.sol
+++ b/test/TwabLib.t.sol
@@ -35,7 +35,7 @@ contract TwabLibTest is BaseTest {
   /* ============ increaseBalances ============ */
 
   function testIncreaseBalance_endOfTimerange() public {
-    uint256 timestamp = PERIOD_OFFSET + uint256(type(uint32).max);
+    uint256 timestamp = twabLibMock.lastObservationAt();
     vm.warp(timestamp);
     twabLibMock.increaseBalances(1000e18, 1000e18);
     vm.warp(uint256(type(uint48).max));
@@ -45,7 +45,7 @@ contract TwabLibTest is BaseTest {
   function testDecreaseBalance_endOfTimerange() public {
     vm.warp(PERIOD_OFFSET);
     twabLibMock.increaseBalances(1000e18, 1000e18);
-    uint256 timestamp = PERIOD_OFFSET + uint256(type(uint32).max);
+    uint256 timestamp = twabLibMock.lastObservationAt();
     vm.warp(timestamp);
     twabLibMock.decreaseBalances(100e18, 100e18, "revert message");
     vm.warp(uint256(type(uint48).max));
@@ -734,7 +734,7 @@ contract TwabLibTest is BaseTest {
   function testGetBalanceAt_endOfTimerange() public {
     twabLibMock.increaseBalances(0, 1e18);
     vm.warp(type(uint48).max);
-    assertEq(twabLibMock.getBalanceAt(PERIOD_OFFSET + uint256(type(uint32).max)), 1e18);
+    assertEq(twabLibMock.getBalanceAt(twabLibMock.lastObservationAt()), 1e18);
   }
 
   function testGetBalanceAt_outOfTimerange() public {
diff --git a/test/invariants/handlers/TwabControllerHandler.sol b/test/invariants/handlers/TwabControllerHandler.sol
index 26c60d2..87b95a3 100644
--- a/test/invariants/handlers/TwabControllerHandler.sol
+++ b/test/invariants/handlers/TwabControllerHandler.sol
@@ -320,7 +320,6 @@ contract TwabControllerHandler is CommonBase, StdCheats, StdUtils {
   function reduceFullRangeTwabs() external view returns (uint256, uint256) {
     uint256 vaultAcc = 0;
     uint256 actorAcc = 0;
-    ObservationLib.Observation memory newestActorObservation;
     ObservationLib.Observation memory newestObservation;
 
     // For Each Vault
diff --git a/test/mocks/TwabLibMock.sol b/test/mocks/TwabLibMock.sol
index 0d96793..f68f06d 100644
--- a/test/mocks/TwabLibMock.sol
+++ b/test/mocks/TwabLibMock.sol
@@ -160,4 +160,9 @@ contract TwabLibMock {
     bool isSafe = TwabLib.hasFinalized(PERIOD_LENGTH, PERIOD_OFFSET, _timestamp);
     return isSafe;
   }
+
+  function lastObservationAt() external view returns (uint) {
+    uint result = TwabLib.lastObservationAt(PERIOD_LENGTH, PERIOD_OFFSET);
+    return result;
+  }
 }