Are the monthly credits that are not used one month rolled to the next ?

No, on a monthly or yearly subscription, your monthly credits will not carry over from one month to the next, even if you haven't used credits at all for one month.

Monthly credits will be reset at the beginning of each billing period.

However, if you have pay-as-you-go (PAYG) credits in addition to your monthly credits, these will not expire within one year and can be used at any time.