How can I add the commission Element to calculate the coverage of the Employee?

Content

In our requirement, the commissioned Employee must have the coverage which is the addition of base annual Salary + Commission earned in last 12 months as their coverage for Optional Life and ADD.

So If I create a Derived Factor which has a Fast formula to fetch the commissions. How can I fetch the Element Entry of an Employee and the values for a particular period of time? 

Can someone share a sample syntax to fetch the Element Entries over a period of time?