The hours stage is virtually identical to the minutes stage (with its "synchronouos discharge" being controlled from the inverted-pin3 output of minutes stage, providing a 1-pulse-per-hour (1PPH) signal), except the "capacitor multiplier" has to provide a 60*12 (or 60*24) multiplication.

This means, that charging current to capacitor drops below 1nA and self-discharge of the capacitor (characterized by its "intrinsic" RC constant of >100ks) becomes significant (12h = cca 50ks). Assuming the self-discharge is consistent, this can be compensated for by increased charging current (decreased R2), but this has to be adjusted experimentally, and there's probably no other way to do this than simply to log the output voltage for the whole 12h/24h, then trim R2, repeat and rinse.