Is there a way to add or subtract to a sim's age by days? I downloaded an aging mod, and now everybody is 6 days into their respective ages!

Get MCcomand it has a function that allows you to set up all ages the amount of days you want.

http://deaderpool-mccc.com/#Top

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. 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. 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.

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. Especially if you play on a long life span where the sims' lives could last for hundreds of days--the bigger the number, especially in modern games, the easier it is to find the value you want. Typically, anyway.

But I haven't messed with it because it requires a bit of work and I'm lazy, and I dunno if TS4 has any sort of basic anti-cheat device protocols in place that would make it annoying, so I just like, y'know, use the potion of life to go back to 0 and make the best of it.

The Wishing Well from Romance Stuff can just subtract a few days, but there is also the chance of a catastrophic failure which could make the situation worse.