For convenience the time grids start at 0. Howver the SSP grids start at 1. This  offset means that the oldest bin in an SFH is set to 0, for both the young and old components. Now rather than trying to interpolate the SFH time grid on the SSP time grid, we rely on the fact that the SFH time grid starts from 0, so it is easy to know which SSP to select (as many as the number of steps). Also adapt the sfh_buat08 module.