Author: Tomas Rutkauskas

How to do date math on calculated fields

Answer:

When doing date math on calculated fields, it is important to ensure that all 
values being used are properly matched as to type. The double method (not in the 
docs) casts the value to a useable type. In the following method, d1 and d2 (part 
of table1) can be of either date or dateTime type and d3 is an integer field.

1   procedure TForm1.Table1CalcFields(DataSet: TDataset);
2   var
3     t1, t2: TDateTime;
4   begin
5     table1d1.asDateTime := Date + 2; {or table1d1.value := date + 2;}
6     table1d2.asDateTime := Date - 2;
7     t1 := table1d1.asDateTime;
8     t2 := table1d2.asDateTime;
9     table1d3.asInteger := trunc(double(t1) - double(t2));
10  end;