Hey Priya! According to GMAC, you can register for the GMAT exam 6 months before the Graduate Management Admission Test date or latest by 24 hours before the GMAT exam date, however, the slot is not available at the last moment. There are no fixed official GMAT dates, you can choose any date according to your convenience and availability. In case you need to retake the GMAT exam you can do so after 16 days. 

Hello Priya,

The filling up of GMAT form for the examination of 2018 will start in the month of August and will continue till the month of December as of now.