Попробуйте,
Было бы проще применить расчетную систему дат к системе, чем рассчитать високосный год.
Private Sub UserForm_Initialize()
Dim y As Integer
Dim i As Integer, s As String
y = 2020 '<~~ your year
For i = 1 To 12
s = Format(DateSerial(y, i, 1), "mmm")
monthListBox.AddItem s
Next i
End Sub
Private Sub monthListBox_Click()
Dim s As String
Dim y As Integer
Dim st As Double, et As Double
Dim i As Integer, cnt As Integer
y = 2020 '<~~ your year
s = monthListBox.Value
st = DateValue(y & "/" & s & "/" & 1)
et = DateAdd("m", 1, st)
cnt = et - st
dateComboBox.Clear
For i = 1 To cnt
dateComboBox.AddItem i
Next i
End Sub
Изображение