Your new interest rate is the daily rate for the day you apply. If rates decrease before you close, some lenders will give you the lower rate. All lenders base their refinance rates on the 10-year U.S. Treasury term plus a spread. Although the changes are slight, mortgage rates change daily. Each lender can charge a different spread and closing fees. For stability, lenders usually lock in your interest rate for 45 days. For most loans, this is ample time for the bank to process the paperwork.