For the billing plan, if you want to get the Settlement from and to date I guess you mean it's periodic billing plan. Is that right?
If so, the from and to date if generated according to the In Advance flag on the billing plan tab header level. In the report, you can simply check the from and to by below logic just like SAP standard screen program does.
If fplt-nfdat < fplt-fkdat, then swap the start and end value to show in the report.
Please check.