use this
string s = DateTime.ParseExact("2130", "HHmm", CultureInfo.CurrentCulture).ToString("hh:mm tt");
TimeSpan span = new TimeSpan(0, 2, 0, 30, 0);
s = DateTime.ParseExact("2130", "HHmm", CultureInfo.CurrentCulture).Subtract(span).ToString("hh:mm tt");
Response.Write(s);