Per diem rates (at least in Europe) change year to year. The tax authorities provide a list of per diem rates for each year. It should be possible to set validity period for a given per diem rate.

Not sure if I got the functionality right, but it seems that I can set the rate for a certain country (and city) but what happens when the rate changes? Is the new rate valid for the new per diem entries only, or does the rate on existing reports change?