Please try the below code. Note that you need to set the value before START-OF_SELECTION
select-OPTIONS: so_date FOR sy-datlo. INITIALIZATION. so_date-sign = 'I'. so_date-option = 'EQ'. so_date-low = sy-datum. CLEAR so_date-high. APPEND so_date.
You can also try this easy option −
select-OPTIONS: so_date FOR sy-datlo default SY-DATUM.