diff --git a/src/app/Wallet.php b/src/app/Wallet.php
--- a/src/app/Wallet.php
+++ b/src/app/Wallet.php
@@ -68,12 +68,12 @@
             }
 
             // This entitlement was created, or billed last, less than a month ago.
-            if ($entitlement->updated_at > Carbon::now()->subMonths(1)) {
+            if ($entitlement->updated_at > Carbon::now()->subMonthsWithoutOverflow(1)) {
                 continue;
             }
 
             // created more than a month ago -- was it billed?
-            if ($entitlement->updated_at <= Carbon::now()->subMonths(1)) {
+            if ($entitlement->updated_at <= Carbon::now()->subMonthsWithoutOverflow(1)) {
                 $diff = $entitlement->updated_at->diffInMonths(Carbon::now());
 
                 $charges += $entitlement->cost * $diff;
@@ -83,7 +83,7 @@
                     continue;
                 }
 
-                $entitlement->updated_at = $entitlement->updated_at->copy()->addMonths($diff);
+                $entitlement->updated_at = $entitlement->updated_at->copy()->addMonthsWithoutOverflow($diff);
                 $entitlement->save();
 
                 $this->debit($entitlement->cost * $diff);
diff --git a/src/database/seeds/UserSeeder.php b/src/database/seeds/UserSeeder.php
--- a/src/database/seeds/UserSeeder.php
+++ b/src/database/seeds/UserSeeder.php
@@ -84,8 +84,8 @@
         $john->assignPackage($package_kolab, $jack);
 
         foreach ($john->entitlements as $entitlement) {
-            $entitlement->created_at = Carbon::now()->subMonths(1);
-            $entitlement->updated_at = Carbon::now()->subMonths(1);
+            $entitlement->created_at = Carbon::now()->subMonthsWithoutOverflow(1);
+            $entitlement->updated_at = Carbon::now()->subMonthsWithoutOverflow(1);
             $entitlement->save();
         }
 
diff --git a/src/tests/Feature/BillingTest.php b/src/tests/Feature/BillingTest.php
--- a/src/tests/Feature/BillingTest.php
+++ b/src/tests/Feature/BillingTest.php
@@ -71,7 +71,7 @@
     {
         $this->backdateEntitlements(
             $this->wallet->entitlements,
-            Carbon::now()->subMonths(1)->addDays(1)
+            Carbon::now()->subMonthsWithoutOverflow(1)->addDays(1)
         );
 
         $this->assertEquals(0, $this->wallet->expectedCharges());
@@ -82,7 +82,7 @@
      */
     public function testFullTrial(): void
     {
-        $this->backdateEntitlements($this->wallet->entitlements, Carbon::now()->subMonths(1));
+        $this->backdateEntitlements($this->wallet->entitlements, Carbon::now()->subMonthsWithoutOverflow(1));
 
         $this->assertEquals(999, $this->wallet->expectedCharges());
     }
@@ -94,7 +94,7 @@
     {
         $this->backdateEntitlements(
             $this->wallet->entitlements,
-            Carbon::now()->subMonths(1)->subDays(1)
+            Carbon::now()->subMonthsWithoutOverflow(1)->subDays(1)
         );
 
         $this->assertEquals(999, $this->wallet->expectedCharges());
@@ -108,7 +108,7 @@
     {
         $this->backdateEntitlements(
             $this->wallet->entitlements,
-            Carbon::now()->subMonths(1)->subDays(1)
+            Carbon::now()->subMonthsWithoutOverflow(1)->subDays(1)
         );
 
         $this->assertEquals(999, $this->wallet->expectedCharges());
@@ -127,7 +127,7 @@
 
         $this->backdateEntitlements(
             [$entitlement],
-            Carbon::now()->subMonths(1)->subDays(1)
+            Carbon::now()->subMonthsWithoutOverflow(1)->subDays(1)
         );
 
         $this->assertEquals(1024, $this->wallet->expectedCharges());
@@ -139,7 +139,7 @@
      */
     public function testAddtStorageLate(): void
     {
-        $this->backdateEntitlements($this->wallet->entitlements, Carbon::now()->subMonths(1));
+        $this->backdateEntitlements($this->wallet->entitlements, Carbon::now()->subMonthsWithoutOverflow(1));
 
         $this->assertEquals(999, $this->wallet->expectedCharges());
 
@@ -163,7 +163,7 @@
     public function testFifthWeek(): void
     {
         $targetDateA = Carbon::now()->subWeeks(5);
-        $targetDateB = $targetDateA->copy()->addMonths(1);
+        $targetDateB = $targetDateA->copy()->addMonthsWithoutOverflow(1);
 
         $this->backdateEntitlements($this->wallet->entitlements, $targetDateA);
 
@@ -181,7 +181,7 @@
 
     public function testSecondMonth(): void
     {
-        $this->backdateEntitlements($this->wallet->entitlements, Carbon::now()->subMonths(2));
+        $this->backdateEntitlements($this->wallet->entitlements, Carbon::now()->subMonthsWithoutOverflow(2));
 
         $this->assertCount(4, $this->wallet->entitlements);
 
@@ -199,7 +199,7 @@
             ]
         );
 
-        $this->backdateEntitlements([$entitlement], Carbon::now()->subMonths(1));
+        $this->backdateEntitlements([$entitlement], Carbon::now()->subMonthsWithoutOverflow(1));
 
         $this->assertEquals(2023, $this->wallet->expectedCharges());
     }
@@ -237,7 +237,7 @@
 
         $wallet_id = $wallet->id;
 
-        $this->backdateEntitlements($wallet->entitlements, Carbon::now()->subMonths(1));
+        $this->backdateEntitlements($wallet->entitlements, Carbon::now()->subMonthsWithoutOverflow(1));
 
         $this->assertEquals(500, $wallet->expectedCharges());
     }
diff --git a/src/tests/Feature/EntitlementTest.php b/src/tests/Feature/EntitlementTest.php
--- a/src/tests/Feature/EntitlementTest.php
+++ b/src/tests/Feature/EntitlementTest.php
@@ -66,7 +66,7 @@
         $this->assertCount(2, $sku_mailbox->entitlements()->where('wallet_id', $wallet->id)->get());
         $this->assertCount(9, $wallet->entitlements);
 
-        $this->backdateEntitlements($owner->entitlements, Carbon::now()->subMonths(1));
+        $this->backdateEntitlements($owner->entitlements, Carbon::now()->subMonthsWithoutOverflow(1));
 
         $wallet->chargeEntitlements();
 
diff --git a/src/tests/Feature/SkuTest.php b/src/tests/Feature/SkuTest.php
--- a/src/tests/Feature/SkuTest.php
+++ b/src/tests/Feature/SkuTest.php
@@ -39,7 +39,7 @@
 
         $user = $user->assignPackage($package);
 
-        $this->backdateEntitlements($user->fresh()->entitlements, Carbon::now()->subMonths(1));
+        $this->backdateEntitlements($user->fresh()->entitlements, Carbon::now()->subMonthsWithoutOverflow(1));
 
         $wallet->chargeEntitlements();