The context of the following conversation is Because Carson had his partner to cover for him, he started work late last night.:
Carson says I had to work really late last night..
The friend responds Why did your schedule get so disrupted?.
Carson responds First it was getting a colleague to cover my shift for a doctor's appointment. Now I'm paying him back..
The friend responds Does he usually work the night shift?.
Carson responds Only one night per week..
The friend responds I haven't seen the doctor in over a year, so I've not had this problem for a while..