You as an individual has decided that 15th would be your last working day, thou it may be 20 days less than the notice period demanded. Also, if the same was not approved in writing that you would be relieved early, then that would be a problem. So, nor you have a leave sanctioned after 15th nor you have a written proof that 15th was accepted as your last working day. Based on this the company has given you the absconding letter.
If, you have resigned and not reporting, then what action you want the company to do, when you yourself are not going the legal way.
But company not consider the same as my last working day as 15th .
In this case can i file legal case against company.
You may take a legal action, but the company has your appointment letter, which will say that you have to server the 30 days of notice. What are you going to do on that front. You have knowingly gone against the rules. So best luck with the legal framework.
The dates that you have written are not what is your last day of working, the dates that the company gives in writing and those that are in accordance with the company policy are the final dates.
So, think the next time, before, you want to cheat the system.
The employer here is behaving in a nasty manner which is uncalled for and undesirable.
If you wish to fight it, then you need to take legal help as the employer is at fault here (I do not say that you are not at fault, you are at fault as you have not served the full notice period).