Оформил: DeeCo
Автор: http://www.swissdelphicenter.ch

function RoundTime(ADate: string; Rounding: Integer; bRound: Boolean): string;
 var
   Year, Month, Day, Hour, Min, Sec, MSec: Word;
   tmpDate: TDateTime;
   Res, Diff: string;
   M: integer;
 begin
   tmpDate := StrToDateTime(ADate);
   DecodeTime(tmpDate, Hour, Min, Sec, MSec);
   if (Rounding > 0) and (bRound = True) then
   begin
     if Min mod Rounding = 0 then
       Res := IntToStr(Min)
     else
       Res := IntToStr(Round(Min / Rounding) * Rounding);
     M := StrToInt(Copy(ADate, Length(ADate) - 1, 2));
     Diff := IntToStr(StrToInt(Res) - M);
     if Copy(Diff, 1, 1) = '-' then
     begin
       Diff   := Copy(Diff, 2, Length(Diff) - 1);
       Result := FormatDateTime('dd.mm.yy hh:mm', (tmpDate - StrToTime('00:00' + Diff)));
     end
     else
       Result := FormatDateTime('dd.mm.yy hh:mm', (tmpDate + StrToTime('00:00' + Diff)));
   end
   else
     Result := ADate;
 end;

 // Example: 

procedure TForm1.Button1Click(Sender: TObject);
 begin
   Edit1.Text := FormatDateTime('dd.mm.yy hh:mm', Now);
   Edit2.Text := RountTime(Edit1.Text, SpinEdit1.Value, Checkbox1.Checked);
   // Example: RoundTime('07.08.02 10:41', '15', True) -- > 07.08.02 10:45 
end;