Calculate time intervals for last visit, relaps and death in a longitudinal study

Dear listers,

I am working on a data for a longitudinal study about kidney cancer. The patients have been operated in 'op_date' and re-evaluated in different time intervals ('last_visit(n)' or 'death_date(n)'). There is no observation for some variables.

I tried to solve this problem with using 'replace' and 'if' however do file is getting bigger and complicated. Is there a better way to do it? Would you show me a starting point for this calculation.

Below you can check a part of my dataset.
[ATTACH]temp_3093_1442260574856_969[/ATTACH]

Thank you very much.
Alp