FF to get element balance for last 24 months and # of months

Summary

For calculating one payroll element need another element balance for last 48 payrolls (24 months)

Content

When calculating one payroll element, I have to obtain another element value for the last 24 month also. Not just value but also number of months.  Employee could be hired more than 2 years but also less then 2 years. I will take average per year and add to yearly salary in order to get daily rate. I am thinking about getting assignment run balance for each payroll in the past. Which DBI can I use or does anyone has sample of code?