Calendar & birthdays don't seem to line up

I've done my first part of the check in a separate save game (normal lifespan & calendar set to 7 days per season).

According to the calendar a baby should auto-age 2 sim-days after birth, (born on Wednesday, birthday on Friday). It is currently sim-Saturday but still no aging, birthday on the calendar has moved to Saturday. On late Saturday the baby auto-aged into a toddler (a day late).

Before we had calendars I don't think this would have been as noticeable.
In your most recent experience the game did not sync exactly with the calendar.
IE: Born on Wednesday (normal lifespan) the game calculates three days later as their birthday.
I am thinking time of day may also be a factor. It (ageing up) could also be dependent on the time of day.
Meaning the day of birth would be considered as day zero and in this instance the third day (aging up day) would be Saturday. 
To me it seems that is the "game logic" and not synced to the calendar for some reason. 
I will be interested to see what you discover in your latest save.

- EQ Avid Sims Player
I do not work for EA
All packs installed and patches are current.
PC / Win 10
Never used CC or mods.

I am sharing this again from our other discussion as a point of reference.

Source Credit:
https://www.carls-sims-4-guide.com/tutorials/sims.php

@107531kop  Remembering your explanation of playing a game with natural aging up maybe consider this.
Do you ever use cake to age any of your Sims?

I propose a workaround of using the cake (for toddlers and older) on the expected date they would be aging up.
Of course clicking on the crib to age up the babies.
Or set up the birthday as a holiday on the calendar your self as a sort of reminder.
Just a suggestion because you should play the game in a way that you enjoy.

- EQ Avid Sims Player
I do not work for EA
All packs installed and patches are current.
PC / Win 10
Never used CC or mods.

Edit to add: 07Dec2019

In the other thread you mentioned you had "removed all mods/cc and deleted the localthumbcache".
Just want to verify you had not put any mods / cc back into your game.

Reference:
"Babies still not auto-aging"
https://answers.ea.com/t5/General-Discussion-and-Feedback/Babies-still-not-auto-aging/m-p/8482524#M19236

Second part of the test is complete.

According to the calendar my toddler was supposed to age up in 7 days (Saturday to Saturday), however it took until Sunday (8 sim-days) before the toddler auto-aged.

As a kid he was sad tho because apparently no-one thought of the idea to throw a birthday party.

Third part of the test completed as well.

One of my sim-parents has aged from young adult to adult. It was supposed to happen on Monday, but happened on Tuesday (again a day late).

The second young adult has auto-aged as well, similar to the other one a day later than the calendar said she would.

Fourth part of the test completed too.

The child-sim has now auto-aged to a teen, again one day later than the calendar set for his birthday.

The fifth part of the test has been completed.

Had a teen become a young adult and an adult an elder.

Both auto-aged a day after the calendar set their birthdays.

@107531kop  So what you are saying is in this test your findings are consistent.
With normal lifespan and the default 7 day seasons the day they age up is one day after it is on the calendar.
Actually thinking back and having consulted with several veteran players this can be expected.
Even with base game and previous versions of the game there has been a notice the day before a Sim ages up.
That gives the player ample notice to decide their next step. Some may want to bake a cake (to avoid the sad moodlet in TS4) or plan a party to celebrate and others like yourself want birthdays to just happen.  It is ultimately up to players to decide how to play out their stories.

With Seasons we now have the added bonus of checking the calendar to be prepared for the upcoming events.
Think of it as the calendar just serving as a reminder and not the actual birthday.
Of course whenever you change the lifespan or the length of the Seasons there may be unexpected changes to holidays and events.
Discovering the rhythm of the calendar compared to the actual event could make your gameplay more enjoyable.

- EQ Avid Sims Player
I do not work for EA
All packs installed and patches are current.
PC / Win 10
Never used CC or mods.

@EuphorialQueenyou beat me to it, but I was going to say the same. When I've finished this test, I'm planning on switching the normal lifespan for a long one and see how that plays out... and perhaps even do one with a short lifespan... I'll keep updating this one with my findings, but with less posts (one post per lifespan I think), so that future Simmers can use it as a guideline as well... (I'll be throwing this save game in the bin anyhow, so why not expand the test with the other settings)