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