[firestore] Use ms instead of ISOString for date conversion

lib/modules/firestore/utils/serialize.js

@@ -67,7 +67,7 @@ const buildTypeMap = (value: any): any => {
       };
     } else if (value instanceof Date) {
       typeMap.type = 'date';
-      typeMap.value = value.toISOString();
+      typeMap.value = value.getTime();
     } else {
       typeMap.type = 'object';
       typeMap.value = buildNativeMap(value);