13. On May 31, 2024, the collective bargaining agreement between Gretel Corporation, Inc. (GCI) and Gretel Labor Union (GLU) expired. After several negotiations, GCI and GLU signed a new collective bargaining agreement on August 31, 2024, which obliged the company to pay a wage increase in favor of the employees. GLU then demanded salary differentials starting June 1, 2024. However, GCI argued that the provisions of the new collective bargaining agreement as to the wage increase shall be prospective in application beginning August 31, 2024. When shall the salary increase be reckoned? Explain.