Basically, higher numbers = slower aging. It’s not super intuitive but the BCON has labels that tell you for each number what days of the week they age so you can remember. 2 = Age only on Thursday and Sunday (twice a week).
Normally that number decreases by 1 for every sim in the active household at 6 PM every night, hence why it’s called “days left”, but with the slow aging controller sims with slow aging (due to their supernatural state or you manually setting them to age more slowly) will only have their days left decrease on certain days of the week. Lifespan mods affect how many days left in their current lifestage a sim has upon aging up - this is one less than the number you see ingame when you scroll over their age bar. The most common type of aging mod, lifespan mods like Hat’s and Nepheris’s, are compatible with BO’s slow aging controller and can be (and likely often are!) used alongside it.
