I have a group of patients that have stopped taking treatment at one health facility (say Facility A) and I am trying to find out the factors associated with transferring to another facility (Facility B) after they stop. I am using a Cox regression model with the starting point being the patient's last visit at Facility A and the endpoint being the first visit at facility B or else the data is censored. I'd like to know if patients are more likely to transfer the longer they stay out of care. Would it be right to include a categorical variable such as time since the last appointment at facility A or is this problematic as it is derived from the outcome variable?