Firstly you need to measure it. Secondly you need to treat everyone the same, or at least make very visible the reasons why some employees get special treatment.

You need to record the times in some way. You need some rules in the staff handbook which you apply fairly and with respect for people’s dignity.

Perhaps you have an IT person who stays late to fix things and comes in late the next day? Make sure people are aware of the reasons for lateness, perhaps by a public thank you at an employee meeting.