I am trying to subtract two times from a SQL data base (ambulance arrive time– dispatch time = response time) and when I check the formula everything is fine (no errors found) but when I run the report I get a "bad time sting format" error

The formula I am using is

CTime ({incident.time_arrived})-CTime ({incident.time_dispatched})

The times are in a 24 hour format

Anyone that can help I would appreciate it.