Update of the 0041 it during the rehiring action

Question

Hi experts!
We have a report which calculates the seniority of an employee (years, months, days he worked for the company). Ofcourse this report uses different dates from 0041.
Currently during rehiring action all dates are set automaticaly to the new hire date, and the previous employment in this company have to be entered to the infotype for previous employers (so that seniority is calculated correctly ).
But the new requirement is that we have to make the system to do this automatically. In other words: if an employee was hired, then fired and, after some time, rehired again (we use the same pers number), the system should know how to calculate the time he spent with the company.
My idea is to do like this:
Instead of setting all dates in 0041 as the dates of new hiring, i want to update the old values.
For example:
Employee was hired on 01.01.2001
Fired on 01.01.2003
Rehired on 01.01.2007
During the dynamic action system should take the old date used for calculating of the time he worked in the company(01.01.2001),
Update this date adding to it the time he was fired (2007-2003= 4 years) and enter the new date: 01.01.2005 as a date wich will be used to calculate the time.
Problem is: i dont know how to do it as i'm not so familar with abap..
Could somebody help?

Answer 1

P.S. We cant modify the report, as it is a standard report for Poland (we avoid modifications).