In newer versions of Access you can make a textbox show a DateTime picker by formatting as a Date or binding it to a DateTime Field and the little Date Picker control will show up.
Or add
Private Sub TEXTBOXNAME_GotFocus()
DoCmd.RunCommand acCmdShowDatePicker
End Sub
If you wanting to have full control over the DateTime picker you could look into using the following:
http://www.599cd.com/templates/access/calendar-control/?key=AlexForum