regarding corporate educational laws

sir recently I joined in one reputed educational coaching institute.all formalities completed.they gave me posting order on one date .but I would like to join on another date .this is due to some commitments with previous institute.i need legal support what can they do if one we refused to join in previous institution in the name of odd timings of that particular institute which is not mentioned earlier.they told first two months will be probationary period.so plz help me