date(urgent)

i have created a program that allow the user to key in the expired date of credit card. if the user key in the date which is outdated ,it will pop up the message .now the problem is i would like to check the month ,day and year is that valid.if i key in the month wrongly(month>12),pop up message"invalid month".if i key in day wrongly,pop up message"invalid date", if i key in year wrongly(<2002)then pop up "Your credit card is expired".if everythings is ok,then
the user press the"summit" button,it will pop up"Thank you for purchasing our product"
but now if i key in the month wrongly, it suppose to pop up one message "invalid month", nut it pop up together with"invalid date!"and "Thank you for purchasing our product"
what's wrong with my code?
here is my code:

private sub cmd_summit_click()
month = Val(txt_month.Text)
day = Val(txt_day.Text)
year = Val(txt_year.Text)

monthvalid = checkmonth(month)
If Not IsEmpty(monthvalid) Then
daynum = day_of_month(monthvalid, year)
End If

If Not IsEmpty(daynum) Then
dayvalid = daycheck(day, daynum)
End If

If Not IsEmpty(dayvalid) Then
yearvalid = yearcheck(dayvalid, monthvalid, year)
End If

End Sub

Private Function checkmonth(m As Date) As Date

If m > 12 Then
Call MsgBox("Invalid month!", , "The Online Shopper")
txt_month.SetFocus
Else
checkmonth = m
End If
End Function
Private Function day_of_month(m As Date, y As Date) As Date

If m = 1 Or m = 3 Or m = 5 Or m = 7 Or m = 8 Or m = 10 Or m = 12 Then
day_of_month = 31
End If

If m = 4 Or m = 6 Or m = 9 Or m = 11 Then
day_of_month = 30
End If

If m = 2 Then
day_of_month = 28
End If

End Function

Private Function daycheck(d As Date, dn As Date) As Date
If d > dn Then
Call MsgBox("Invalid date!", , "The Online Shopper")
txt_day.SetFocus
Else
daycheck = d
End If
End Function

Private Function yearcheck(d As Date, m As Date, y As Date)
If y <= 2002 Then
Call MsgBox("Your credit card is expired!")
txt_day.SetFocus
Else
yearcheck = y
End If

End Function