From Unofficial BOINC Wiki
In the Climateprediction Models, a Time-Step represents a 1/2 hour of model time (not real-time). Every 10,802 Time-Steps your model will Trickle (report), and there are 24 Trickles in a phase. When you know that a model year is 12 months of 30 days (not 365 days), you can work out that a Phase is 15 years 1 day of model time (not real-time).
So, a Time-Step is a 30-minute time-slice of processing for many parameters, in Model Time, for the entire atmosphere. Every sixth Time-Step requires more time because there are added heat-flux calculations -- short-wave solar radiation in, long-wave terrestrial radiation out.
Until the Coupled Model goes public, the ocean is simplified and called a slab. The slab can act as a heat source or sink which can to some extent simulate the effect of currents flowing. However, there isn't any real current nor turn-over (vertical motion) in the slab oceans so while a slab model can simulate a steady current it cannot simulate a dynamic change. This goes a long way to explaining "Cold Equator" Runs. Consequently, Time-Step are atmospheric repetitions through time for the parameter set for the particular Model and Phase.
Each Time-Step processes 7,008 grid cells per level, for each of 19 atmospheric levels. (73 Latitude by 96 Longitude) Of course, the cells are not treated in isolation. Calculations must consider what occurs in surrounding cells, in both the horizontal and the vertical.
There are 259,248 Time-Steps per Phase (15 Model years), organized into 24 groups of 10,802 Time-Step per Phase. (Three Phases for HadSM3, five Phases for Sulphur Cycle Model.) After each 10,802 Time-Step, a "Trickle" is sent to the server. These Twenty-four Trickles do little more than tell the servers that the Model is alive and well. There is another mega-trickle (or GM trickle) sent after each phase which contains summary information for your Model and is used to generate the graphs you see on the Project Stats page and in the "Your Account" Page.
A lot of work is done for each Time Step.
No wonder it takes so long, eh?