There is a bug in Local SQL on Paradox:
Executing an Update statement on a Paradox table where '1' is being subtracted in a datetime field does not subtract '1', but rather adds '1'.
// this will ADD one!
UPDATE SAMPLE.DB SET DT = DT - 1
// the following workaround will give the correct result:
UPDATE SAMPLE.DB SET DT = DT + (-1)