Imputation
This variable is equal to TRODate except in the following cases:
1/ if TRODate is issing, TRODateComputed is equal to VisitDate
2/ if TRODate>VisitDate (the generated date is after the trial status date), TRODateComputed is equal to VisitDate
3/ if the exact year is unknown in TROVPDate, TRODateComputed is equal to VisitDate