Solution #1 is very good and shows a way how to deal with date formats.
As to your code, check this:
Void t_Tick(object sender, EventArgs e)
{
Timespan tt = end.Subtract(DateTIme.Now)
int sec = tt.Seconds;
Tb.text = string.Format({0} seconds left", sec);
}
For further details, please see:
TimeSpan.Seconds Property (System) | Microsoft Docs[
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