I have a universal function that resolves this problem for all versions of Fox.

The function (BASESECS()) takes two parameters DATE and TIME
I calculate the seconds difference between the DATE/TIME parameters passed and a "base" value (one that is significantly earlier than any date I would be dealing with in my business).

Then calculating the difference in seconds between two date/time combinations is simple the differnce between two calls to the BASESECS.

Of course it is much easier in a homogenous VFP environment using DATETIME variables as Ed has pointed out.