In your resignation letter should be written a date. Also resignation letter is start effect when u tender it to your HR department (if no HR department then is your boss).
In EA state "the notice only effective from the date of submission of such resignation" section 12(4).
If your office got CCTV, try to retrace back the moment you tender the resignation. If don't have CCTV, try to find witnesses.
If both also don't have, then can't do anything (you and your boss) because your boss also no proof that you show you tender your resignation on 18th mac.
Personal opinion: If i were you, i will continues work until 18th April then all thing will settle, Your boss much pay your salary up till 18th April. If got AL then take all.
Go some clinic buy some MC and give to your boss. Holiday + paid + claim (if you want do it in the bad way) <-- not encourage unless your boss really bad =.="