The logbook says "at commencement" (i.e. at the time this logbook starts from, the engine has 0 hours). It's because there might be multiple logbooks for an eng

The logbook says "at commencement" (i.e. at the time this logbook starts from, the engine has 0 hours). It's because there might be multiple logbooks for an engine, so if you reached the end of logbook1 and it had 5000 hours accounted for, logbook2 would say 5000 hours at commencement.