fix(spanner): correct chrono arithmetic in Interval integration test (#15163)

Author: prathmeshcc

google/cloud/spanner/integration_tests/data_types_integration_test.cc

@@ -535,7 +535,9 @@ TEST_F(DataTypeIntegrationTest, SelectIntervalFromTimestampDiff) {
           std::chrono::hours(1))};
   std::time_t now_seconds =
       std::chrono::system_clock::to_time_t(std::chrono::system_clock::now());
-  std::time_t one_hour_later_seconds = now_seconds + 3600;
+  std::time_t one_hour_later_seconds = 
+      std::chrono::system_clock::to_time_t(
+          std::chrono::system_clock::now() + std::chrono::hours(1));
 
   std::vector<std::vector<Timestamp>> const data = {std::vector<Timestamp>{
       MakeTimestamp(MakeTime(now_seconds, 0)).value(),