Calculating the difference between two dates

I need to calculate the difference, measured in days or weeks, between two dates.  Can this be done using AdvStringGrid or AdvSpreadGrid?   The date/time functions for the AdvSpreadGrid seem rather limited.  Any assistance will be appreciated. 

I tried the demo AdvSpreadGrid and it calculates the difference in days, just like I want.  Can this be done with the AdvStringGrid (which has no built in formula functions) or should I replace my AdvStringGrid with the AdvSpreadGrid and do the calculations programmatically?

There is currently unfortunately not a built-in function to do this.

I'm not sure how exactly you'd want to do this calculation. Is this a fixed calculation or does the user need to specify how to do this calculation? With the first, you can programmatically do this by using Delphi's date functions applied to grid.Dates[col,row]: TDateTime.
When this needs to be end-user configurable, you could implement a custom function library in TAdvSpreadGrid to do this. In the unit MiscMathLib.pas you can see several sample custom function libraries implemented.