In the detailed host stats there's a 'Average credit per CPU second'. Is this value an all-time average or a recent average?

If it's indeed an all-time average, then the total CPU-runtime for that host can be calculated by deviding the total credit by the average credit per CPU second right? And doing so for all my hosts I would have my total Boinc CPU-time.

I guess there's some reason why this doesn't work, otherwise it would have been added to the statistics long time ago...