The concept is correct. The allocation of bandwidth allowance is based on the hours the Droplet is active ( meaning created/existing, which is apart from powered on or powered off state ) on the account.

Because the actual hourly accrual is less than 30 days ( based on a 672 hourly cap for the Droplet itself ), the exact math results in 15 days being slightly more than 500GB but overall that is the correct idea. If the Droplet is destroyed before the full 1TB is accrued within a month, any excess outbound public traffic is billable at the bandwidth rate.

Keep in mind that if you have more than one Droplet, the total allocation of bandwidth allowance is increased for the account, so the bandwidth might not be billed if the other Droplets have a net positive amount of allowed bandwidth transfer pool for the account.

The accrual and billing is outlined here for reference:

https://www.digitalocean.com/docs/accounts/billing/bandwidth/