There is a great post in mangahelpers forum about timeline.I am posting it here….
Eigengrau said: ↑Yeah, I guess lying was the correct thing to do at the time. Hell, next month we will probably see more of Krueger/Grisha, and Krueger might be the one to tell Grisha what to do : "infiltrate, learn, take your time".
As for the timeline, I've been looking at things and well, here's what we know:
850: present time, Eren is 15 years old;
845: when the story starts, and when Shiganshina/Wall Maria is first breached. Eren is 10 years old;
This means that Eren was born in 835.
In 850, Shadis tells them that he met Grisha 20 years ago = 830.
This means that Grisha arrived in the Walls in 830, and he was there for 5 years until Eren was born in 835.
We also know that 845 = 100 years of peace/100 years since the Walls were built.
When the Marley started recruiting, it is explicitly said that it's been 80 years since the King fled and created the Walls, so the Marley recruiting started in 825.
Now, this is where it gets tricky, but I believe we have found our answer:
In 825, Zeke was already alive, so the question is, how old was he back in 825? Was he 7 years old when he was recruited AND betrayed his parents? Or was he recruited but grew up for a few years before betraying his parents at age 7?
I believe that in 825, Zeke was 2 years old.
Remember this? We are told that after he was born, "one age turned to another".
So when they decide to turn Zeke into a Warrior, in 825 he is 2 years old.
He proceeds to be brainwashed by the Marley for 5 years, until 830 where, finally, at age 7, he betrays his parents, and have them sent to the island and turned into Titans, where Grisha immediately becomes a Shifter and ends up in the Walls.
I think it all fits.
So if Zeke was 7 years old in 830, now, 20 years later  currently 850  he should be 27. Not sure how I got to 3233 years old before, but this seems to be right… right?
EDIT: oh right, I remember: if Zeke was 2 in 825, he is now 27, like I said, but if he was already 7 at the time, he would be 3233 years old, but I think 27 is more likely now.
In short:

825 > Zeke is 2 years old, begins his training as a Warrior;

830 > Zeke, now age 7, betrays his parents;
> Grisha and everyone else is sent to the island;
> Grisha becomes a shifter, Shadis finds him;

835 > Eren is born;

845 > first breach; Eren is 10 years old; Grisha dies/is eaten by Eren;

850 > present time, Eren is 15 years old.
Grisha lived for 15 years inside the Walls  he was only a father for the last 10  and it's been 20 years since Zeke (7 years old) betrayed his parents. Zeke was born in 823, which makes him 12 years older than Eren.
I don't think there's any "missing gap" for Grisha, is there?