Последний день месяца возвращается DateTime.DaysInMonth () , передавая текущий год и месяц.
Dim FirstOfMonth As Date = New DateTime(Date.Now.Year, Date.Now.Month, 1)
Dim EndOfMonth As Date =
New DateTime(Date.Now.Year, Date.Now.Month, Date.DaysInMonth(Date.Now.Year, Date.Now.Month))
В аналогичном формате, примененном к вашим элементам управления:
DtpIssue.MinDate = New Date(Today.Year, Today.Month, 1)
DtpIssue.MaxDate = New Date(Today.Year, Today.Month, Date.DaysInMonth(Today.Year, Today.Month))