This is a .Net forum, yet you appear to be using the old VB
style methods. These are in the compatibility layer, but things are usually much easier if you use the newer 'pure' .Net classes.
The following will give you the last day of the current month stored in newdt variable:
Newdt = new DateTime(Newdt.Year, Newdt.Month, 1).AddMonths(1).AddDays(-1)