My question involves labor and employment law for the state of: Arizona

My wife recently received a pay raise for obtaining an additional license that began on the first of the month. The contract that her employer gave her to sign said that her pay increase would begin on the first of the month, however their HR department was slow and she didn't receive the hard copy of the contract to sign until the 16th (after the first pay period). Does her employer owe her the increased wages for the first pay period of the month (as stated in the contract) or is she out of luck because she signed it after the first pay period? Thanks for any help!