Be Careful of Communicating Working Hours with your Boss

You can't take advantage of your boss's tolerance.
I am not sure about the nature of your job. If you have a 9-5 job in Corporate, your producitivity could still be good even if your schedule is now down to 9:15 to 5.
However, when you mention to your boss that you might be coming late, you should also offer that you will make it up - such as change your schedule from 9-5 to 9:15 to 5:15. Otherwise, you will leave other's impression that you just want to work less hours, especially that you keep coming late since then. Your boss would think you are changing your own work schedule from 9:15 to 5 and therefore cutting your normal working hours. He may still be satisfied with your productivity, but he may also think that you are not giving 100% at work.