How to make date and time as blank instead of '00000000'?

Dear firends,

I have two fields as two columns in my cl_gui_alv_tree, the default value are sy-datum and sy-uzeit. However, sometimes there are no values for them, in my cl_gui_alv_tree, it will displayed as like '00000000', but my requirement is to display them as blank, I have used clear command, set it to the space or ' ', but it always display like '00000000'.

So how to make the empty date and time as blank instead of '00000000' for the display?

Thanks a lot!