Your data plan price will be prorated for your first month based on your activation date. However, data allowance is not prorated, meaning you’ll receive the full monthly data allocation regardless of your activation date.
To better understand your first bill, please refer to here.
For the last month of service, neither the price nor the data allowance is prorated. If you terminate your plan partway through a billing cycle, you’ll be charged the full month's fee and retain access to your full data allowance until the end of that cycle.