private void spin_ValueChanged(object sender, EventArgs e) {
            intervalChanged(spinMinutes, spinSeconds, spinMilliseconds);
            lblInterval.Text = "Interval (milliseconds): " + Interval(spinMinutes, spinSeconds, spinMilliseconds);
        }

        private void spinSeq_ValueChanged(object sender, EventArgs e) {
            intervalChanged(spinSeqMinutes, spinSeqSeconds, spinSeqMilliseconds);
            lblInterval.Text = "Interval (milliseconds): " + Interval(spinSeqMinutes, spinSeqSeconds, spinSeqMilliseconds);
        }
        #endregion

        private bool intervalUpdating = false;
        private void intervalChanged(NumericUpDown m_, NumericUpDown s_, NumericUpDown ms_) {
            if (intervalUpdating)
                return;

            intervalUpdating = true;

            // adding time
            if (ms_.Value >= 1000) {
                var val = (int)ms_.Value / 1000;
                s_.Value += val;
                ms_.Value = (ms_.Value - (val * 1000));
            }

            if (s_.Value >= 60) {
                var val = (int)s_.Value / 60;
                m_.Value += val;
                s_.Value = (s_.Value - (val * 60));
            }

            if (m_.Value >= 60) {
                m_.Value = 60;
            }

            // subtracting time
            if (ms_.Value < 0) {
                s_.DownButton();
                ms_.Value = 999;
            }

            if (s_.Value < 0) {
                m_.DownButton();
                s_.Value = 59;
            }

            // warn if too fast
            if (Interval(spinMinutes, spinSeconds, spinMilliseconds) < 250) {
                MessageBox.Show("Intervals below a quarter of a second are strongly discouraged.", "Error", MessageBoxButtons.OK, MessageBoxIcon.Warning);
                ms_.Value = 250;
            }

            intervalUpdating = false;
        }