Problem with replacing a value in a listbox

Ok my listbox starts out with all "none" values. Then when the user selects one of those values and hits a button. I want that value to chnage to the currrent time in the list box. I can only get it to add any new value to the bottom of the list. How can I get it to replace the selected value?

if list1.listindex <> -1 then
list1.list(list1.listindex) = cstr(Now)
end if

Charlie Zimmerman
http://www.freevbcode.com

Thank you so much, that did the job.