How can a sim age up every year on the same day?

A sim year is 28 days unless you have changed the lengths of your seasons.

@mightysprite I did. 14 days and I've passed one sim-year already. I'm on Halloween of the second rotation of the last day of fall.

It seems all you need to do is add together the number of days in each of your four custom seasons, and there you have the length of one year in your game? Unless I'm misunderstanding your question.

@mightysprite there are 56 days. That's all I know. What I don't know is how to calculate the number of days to make each age span last a year on Long age span.

With MCCC, setting your lifespan to long multiplies the individual life stage lengths by 4, while setting it to short will halve them. You could either leave the lifespan on long and set each stage to 14 days, or switch to medium and set them to 56.

I would just turn aging off and use the birthday cake to age them up on their birthday.

@HillyBeth Ah ok. Yes, you would need a mod that allows you to tweak each life stage individually. The regular lifespan stages are not the same as each other. The baby stage is very short, the toddler stage is longer but still much shorter than young adult, etc.

The simplest solution is probably @babajayne 's suggestion, turning aging off and using the cake on the proper day, or bassinet for babies (you can still leave aging on for NPCs if you want to, that's a separate setting.)

Another thought- you probably don’t want to age the adults at the same rate as their kids. They’ll end up dying of old age before the kids are grown. I’d recommend that you skip their birthdays every other year or something like that.

I just guestimate the age of my Sims when I tell their stories. But I will have to figure out just how old my Sims are. I plan on using aging off when the now Teens age up to YAs and head off to Uni. Three Terms is how long? Since the Teens are all about one Sim-day apart in age, should I count each birthday as one year? (I'm sending them all at the same time to uni.) So that by the time the youngest teen is a YA, three years has elapsed? With three Terms does that equal six years including the time they all aged up? (YA for me begins at age 18 as it does here in the U.S.) So that the second Teen is twenty-three by the time of graduation? In my mind the first two are about eighteen months apart in age. They all came boom, boom, boom and boom. Only the youngest kid in that household is much younger. He's still a Child until the second eldest kid becomes a YA and the Child becomes a Teen. Interesting how the game gave them the exact same birthdate.

I would count the seasons, all four to make a year, in your case. You can change your Sims' birthdays, too. Just created a Birthday party for them and voila. They will age up when you choose. Generally, I have done this to set birth order, but I have since been able to fix that via MCCC, instead.

Is there no such mod that allows you to extend the season length past 28 days? In the Sims 3 you could turn the season length off.