Pursuing PhD, got a job offer : company wants to file H1B petition: graduation date uncertain

Hello
I am currently pursuing PhD in US, my tentative graduation date is Aug 2015, last semester I got a job offer from a company in US.
Recently, I received an email from my employer that they want information for filing for my H1B petition.
Currently my joining date is set as Sept 2015, however I will only be able to know by March 2015 (committee review meeting) if I am graduating in August this year or in December.

If my employer files an H1B petition for me stating my joining date as Sept 2015 and I am granted H1B visa, how will this affect me if my graduation and hence my date of joining is delayed to December ?

Will it have any adverse effect ? Or is it ok for the company to file the H1B petition ?

Thank you.