Meenakshi,
you can create a internal table copy to access the previous task and use case to check the condition.
it_swpsteplog_pre[] = it_swpsteplog[].
* Read selected entries
loop at it_swpsteplog.
Check returntype values
CASE it_swpsteplog-returntyp.
* Returntyp = RC
WHEN 'RC'.
"your logic about Start Date/Time
* Returntyp = EV
WHEN 'EV'.
"your logic about Completion Date/Time
"read your auxiliar internal table it_swpsteplog_pre
"to get previous task (example using logic key... or
"using sy-tabix logic...)
ENDCASE