cox proportional hazard model time variable

[lillymaginta]

id drug  end_Date   enr_Date o_date
1   a      01/02/2005   04/02/2004  01/03/2003

I have the following database and I am trying to run cox regression. My question how to code the time component as day to the occurrence to the first of either end_Date, enr_date or o_date? 

Thanks