In this code, you have used ‘BT’ i.e. between so select option will hit any date between today and YYYYMMDD ‘99991231’.
You should declare high dates at INITIALIZATION so that it is visible on the selection screen and you can change it if required.
select-OPTIONS: so_date FOR sy-datum. INITIALIZATION. so_date-sign = 'I'. so_date-option = 'BT'. so_date-low = sy-datum. so_date-high = '99991231'. APPEND so_date.