While playing the TS4 settler challenge and trying to keep sims aging in sync with the real world timeline, I came up with a different concept of the passage of time for my sims. It´s very basic, as works best for me, for example I didn´t delve deeper into creating the exact solar system that would offer the conditions described. But perhaps some of you might like to build upon my skeleton for your own games? They system doesn´t work for games that portray our reality, however.

Here it is:

1 Ingame (IG) Year = 1 Story (S) Year
1 Ingame Month = 1 Story Quarter / 3 Story Months (30 days)
that makes 1 Ingame Day = roughly two Story Weeks (18 Story days)

One year consists of 4 seasons with 90 "suns" (5 IG days) and 90 "moons" (5 IG nights) each. The naming stems from a time when people believed that the sun would lay down to die at the end of each day and the sun that comes up the next morning subsequently was a new one.
The combinations of time of time of day and season a person is born are handled akin to our zodiac signs – sims try to derive information about the character, strengths and weaknesses of a society member born during that time. There´s no future prediction element other than „people like kinda excel in xy and most likely to reply with reaction b to event a“ inherent in the sorting.

A year has 360 Story Days and begins at the first day of whatever season you choose. (I´m not sure if the New Year Party option is tied to the Winter/Spring season or to the days in late Season 4 and early Season 1). Thus seasons never carry over to another year, though the actual weather might not care about that fact…

Weeks are a very recent invention. The days of the week were defined during the industrial revolution to better manage the workload and in an attempt of the old government to guarantee the workers some freetime. Most sims never really accepted weeks, as they do not match well with the traditional calendar. It´s one of the annoyances only school aged sims have to cope with.

On average individuals live 90 (?) IG days, aka 4 or 5 years. This is their natural lifespan, free of fiddling with equations or mods.
During the elder and/or adult lifestage the Elixier of Age can add up to another year to an individual´s lifespan without drawing unwanted attention to that sim. Prolonging the child and teen lifestages isn´t recommened, as it may slap the young sim with the late bloomer or even mentally ill stigma.
The elixier of life is not acknowledged in the story (except when it is produced by a cow plant). The sims will use it "offstage" and other sims will comment along the lines of „Wow, for her age she´s very fit!“. An exception is elixir produced by a cowplant. This is always real and WILL draw the attention of law enforcement and scientists.

Gradual aging is unknown. The Sims are aware of the lifestages and experience the transition as something perfectly natural. ("No, silly, it doesn´t hurt." "But John said if I wasn´t careful during the spinning, my innards would get tied up in a knot and then I´d DIE!")
Birthdays are tracked seperately from aging up celebrations. Especially the first birthday is very important for the young teen. It marks the day when they are allowed to drive, perform romantic socials, drink, smoke etc. If your sims are on free will (or perform activities they are not yet allowed to as visitors on community lots where you cannot hinder them) then free to come up with fines or other forms of punishments.
The six-days-from-agingup message is another milestone. Teens may now leave school and enter the military or another part time career (or just enjoy their freetime). They are considered of age now, even though they are still unable to procreate.
Depending on the society these two milestones may require the teen sim to perform certain rites or pass some sort of test. It doesn´t have to be a „primitive“ tribal society. My Hillsfar neighborhood for example features a high technology nation that requires their young members to live like their ancestors did for a given period before they are allowed to vote or take up careers.

Can´t wait to see how it plays out, but for now it´s back to 1844 and calculating my settlers.