Опираясь на ответ, который я дал на ваш последний вопрос ...
Dim dt As Date
Dim DaysInLastMonth As Integer
Dim LastMonth As String
dt = CDate(txtPeriod.Text)
dt = DateAdd("m", -1, dt)
LastMonth = Format$(dt, "mm/yyyy")
DaysInLastMonth = DateDiff("d", dt, DateAdd("m", 1, dt))
MsgBox LastMonth & vbCrLf & DaysInLastMonth