The `_calculate_vested_amount` function is called in ` claim` and `claimable_amount` functions. There is a check that ensures the `block.timestamp` is greater or equal to the `vesting_start_time` in the both functions. Also, the admin sets the start and end time of the vesting. This means it will be always correct. Therefore, there is no risk from underflow or division by zero in `_calculate_vested_amount` function.