Calculation in User Form

Add a ComboBox to the Form, a DatePicker Control & I would suggest a Label with a white background. Then this code

Option Explicit

Private Sub ComboBox1_Change()
    Dim dte As Date
    With Me
        dte = .DTPicker1.Value
        dte = DateSerial(Year(dte) + Val(.ComboBox1.Value), Month(dte), Day(dte) - 1)
        .Label1.Caption = ""
        .Label1.Caption = Format(dte, "long date")
    End With
End Sub

Private Sub UserForm_Initialize()
    Me.ComboBox1.List = Array(1, 2, 3)
End Sub