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Databases in general require UNIX timestamps when reading/writing a time field. TDateTime is specific to Object Pascal. What if you want to save a date value in your applications database? Well, convertions (back and forth) are quite simple. The code is included below.

    const
      UnixStartDate: TDateTime = 25569.0; // 01/01/1970

    function DateTimeToUnix(dtDate: TDateTime): Longint;
    begin
      Result := Round((dtDate - UnixStartDate) * 86400);
    end;

    function UnixToDateTime(USec: Longint): TDateTime;
    begin
      Result := (Usec / 86400) + UnixStartDate;
    end;

You can find very similar routines (DateTimeToUnixDate & UnixDateToDateTime) in the DelphiDabbler Code Snippets Database.