Code:

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;