You can also re-write your code like this :
private static readonly DateTime UnixEpoch = new DateTime(1970, 1, 1, 0, 0, 0, DateTimeKind.Utc);
private void timer1_Tick(object sender, EventArgs e)
{
label3.Text = DateTime.Now.ToLongTimeString();
long dte = (long) (DateTime.UtcNow - UnixEpoch).TotalMilliseconds;
long dts = (long) (dateTimePicker1.Value - UnixEpoch).TotalMilliseconds;
if (dte == dts)
MessageBox.Show("gsd");
}