The baseyear database may contain data from before the base year. If you want
this data to be available to the simulation, put it into a lag table whose name
is the same as the non-lag table for that dataset except with _lag at the
end, e.g. gridcells_lag, and that contains a year column indicating
the year for each row. The first step of a simulation run copies this lag
data into the baseyear cache, so that it can be accessed by lag variables.