How is the computing time calculated

Running jobs at CSC's servers uses computing saldo. If that runs out, you can apply for more (link)

The billing is calculated based on the resources that have been granted to your batch job. For example, if you ask for 24 hours and 4 cores, but the job ends after two hours, your project will be billed 2 x 4 core hours. Even if you ask for 24 hours from the queuing system, you will not be billed 24 hours as the resources become available for other users when your job ends. Note, that the billing is not based on actual CPU usage, so if your code is using only little CPU, e.g. due to inefficient parallelization, all resources that have been allocated to your job will be billed.