My background is engineering nothing upsets me more than incorrect dates. I really don't think getting dates corrected is a tough call. They shouldn't have his birth date marked incorrectly in their books because he is a character that shows up in games and books.
Well, like I said, Gaider almost certainly intended for Fiona to be Alistair's mother. The problem was that the Calling had a very short editing cycle so he wasn't able to get all the dates right.
And I understand that as lead writer he doesn't control every line of dialogue in the game, but given that he was writing for Alistair I think he could have coordinated with whoever was writing for Loghain and Eamon or anyone in a position to know Alistair's backstory to at the very least, get the general time frame right.
And I assume his writing for Alistair included Alistair's personal quest involving Goldanna; the problem is that that quest makes no sense when combined with the Fiona story. The whole point of keeping her kid away is that he wasn't supposed to know who his parents were. It makes no sense that they would tell him, or tell this girl who's mother just died.
The whole thing is entirely too convoluted and utterly pointless. If you want to keep Alistair away from the throne you have Eamon bite the bullet and claim to be the boy's father. I'm decently sure Eamon wasn't married at that point so it wouldn't have caused too many problems for him.