diff --git a/backend/__tests__/fp-submissions-cleanup.property.test.js b/backend/__tests__/fp-submissions-cleanup.property.test.js
index 12cb164..6027ff3 100644
--- a/backend/__tests__/fp-submissions-cleanup.property.test.js
+++ b/backend/__tests__/fp-submissions-cleanup.property.test.js
@@ -31,7 +31,9 @@ const lifecycleStatusArb = fc.constantFrom('submitted', 'approved', 'rejected',
 
 const dismissedAtArb = fc.oneof(
     fc.constant(null),
-    fc.date({ min: new Date('2020-01-01'), max: new Date('2030-12-31') }).map(d => d.toISOString())
+    fc.date({ min: new Date('2020-01-01T00:00:00.000Z'), max: new Date('2030-12-31T00:00:00.000Z') })
+        .filter(d => !isNaN(d.getTime()))
+        .map(d => d.toISOString())
 );
 
 const submissionArb = fc.record({