Yes, ADD-ON BW rollover for up to 12 months, but your base BW expires at of the month regardless of how much you have consumed. Base BW is the minimum BW which you purchased per month, it starts from 1 TB per month.
The ADD-ON BW roll happens on the 7th of every month after consolidating all usage. You can reach out to us anytime hence if any issue we will credit the unused Add-On BW to your account!