Sorry my late to response... some fix to design tasks on theme, and I forgot the hours.
I'm using datetimepicker, so I needed firstly to backup my code, for not mess up, with remove/add code, like I did before (roll back the code is not good, backup is better).
Don't worries, I know what I did to test only with datepicker, and now I testing only with that without any other commented code to datetimepicker, was removed.
You are right, the name of my fields is s_startdate, so I changed that now on php function.
I use two ID for two fields, one on header to homepage, and another on sidebar. So exists different header on both pages for not repeat content of forms.
The results to s_stardate "asc" working now
. The only problem like you said before is, if user click on another sort by, he cannot reverse to date, unless he click on datepicker again.
So I assume not exists solution to do it, to add new type "sort by" on menu.
Unless some trick to do it.
Anyway I'm happy to have this part you done already solved
Thanks teseo, nice work!