There should be 1 general Date Format at all for how FM writes the data internally, regardless the OS Systems preferences been set!
I just had to rebuild a time-period work around with a relation which failed with 1 out of 50 PC users due to inconsistencies with key fields
Table_A::f_Periode = Table_B::Date
I had to rebuild relationship to
Table_A::beginn ≤ Table_B::Date
Table_A::end ≥ Table_B::Date