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;