Indian finance minister will present the union budget on Monday, 29 February 2016.
He can not shift to GST rates before passing the GST in both houses of the parliament.
So he has to present the budget for FY 2016-17 in accordance with the existing tax system. That means, the system will continue for some time.
Therefore even if the parliament passes the legislation on GST, the specific date of implementation may not be possible in the Q1 of the coming fiscal year.