I think this just means that the record with the date you are trying to find isn't currently selected. If a record is currently invisible, it can't be find. Just add a
selectall before the
find and you'll probably be fine.
Also, the second to the last line
isn't necessary. It doesn't hurt anything, it's just an extra step that isn't doing anything useful for you (unless you have some other reason to want Start_date to be the current field.