AlarmForm1.cs

using System;
using System.Windows.Forms;

namespace Alarm
{
public partial class Form1 : Form
{

public Form1()
{
InitializeComponent();
}

private void Timer1_Tick(object sender, EventArgs e)
{
DateTime now = DateTime.Now;
labelNow.Text = now.ToString("HH:mm:ss");//変更（ｈｈ＝12時間制：ＨＨは24時間制
if (now.Hour == hour && now.Minute == minute && flag == true)
{
flag = false;
labelAlarm.Text = "00:00";
labelAlarmFlg.Text = "アラーム未設定";
MessageBox.Show("時間ですよ！", "アラーム", MessageBoxButtons.OK, MessageBoxIcon.Information, MessageBoxDefaultButton.Button1, MessageBoxOptions.DefaultDesktopOnly);
}
}

private void Form1_Load(object sender, EventArgs e)
{
labelAlarm.Text = "アラーム未設定";
}

private void ButtonSet_Click(object sender, EventArgs e)
{
Form2 form2 = new Form2();
form2.alarmFlg = flag; //追加：重要：form1で保持していた値をform2に渡した
if (form2.alarmFlg)
{
form2.hour = hour;
form2.minute = minute;
}
if (form2.ShowDialog() == DialogResult.OK)
{
hour = form2.hour;
minute = form2.minute;
labelAlarm.Text = hour.ToString("00") + ":" + minute.ToString("00");
labelAlarmFlg.Text = "アラーム設定中";
flag = true;
}
else
{
labelAlarm.Text = "00:00";
labelAlarmFlg.Text = "アラーム未設定";
flag = false;
}

}
}
}