One way to do this would be have an entry called Open and have it set to 0 then make a cron job every 24 hours to set it to 1, then once someone submits it will set it back to 0 and it always checks it before it allows then to submit.
Then if its 0 it can display the error message and if its 1 itll say the success message. You can also store the time until the cron job goes again and have that for the countdown timer. Should be easy enough to code, may have to search google about the cron job but the rest is simple queries. Hope it helps.