Sadly, I don't think there's any mod that lets us add or subtract days whenever we want, but I've often thought it would be an easy project to undertake in something like Cheat Engine, a PC application for sniffing out certain in-game values and changing them to whatever you want, or freezing them. Alternatively, you could spend 1,500 satisfaction points on the potion of youth (or whatever) to accomplish the same feat, but that's a part of the game already, without mods. You can change their age state on the fly, though, so you could age them up instantly and back down or vice versa and it should reset them at 0, but that's the only way I know to manipulate the current number of days.
Yeah, this mod is absolutely essential, however it doesn't allow you to add or subtract days from a sim's current lifespan, as far as I know. Get MCcomand it has a function that allows you to set up all ages the amount of days you want.
