If i were to do it again, i'd try a RCfilter and an OP(comparator) connected as a schmittrigger with as big hysteresis i possibly could get away with. Actually i'd choose a pic with comparators onboard. This was not an option for my old clock, pics weren't around back then. Ahhhhh, the good old times ..... NOT. Pics make life much easier, atleast when compared to using 74 and/or 4000 chips.