On the birth dates, I also think it's questionable that a character would celebrate a birthday in a chapter. My opinion is that shonen characters never do that, instead their Mangaka decides to have a time-skip, and the new ages for all characters are announced. That said, the actual age of the characters is VERY important, even though the actual date is BS. In the past, I've posted a thread about the possibility of a timeskip in OP, but to avoid being crucified I'm not even gonna go there. This concerns me greatly because I SERIOUSLY DO NOT WANT LUFFY TO BE PIRATE KING @ 17yr of age.
Back on topic, I don't think 3 months and 2 months makes that big a difference since both can work. Oda has left the story flexible that way (a genius move, if you ask me, for not digging himself into a hole by leaving certain things vague, while giving fans all the definitive facts to chew on). Like I said in my second post of this thread, the maximum voyage length, with all the "several-day-gaps" considered, is probably around 71 days. If Jobebob had his way, maybe it would be 81 days instead (closer to 3 months). Big difference? Meh…
I think the only important argument in this heated debate is whether or not Oda really has a time frame in mind, and whether or not it's consistent, and affecting the story.
--Starting backwards, I would say it's not affecting the story much. Why? Because most OP fans haven't even noticed it, and the manga is just as enjoyable as it ever has been. So maybe some aspects are less believable; that's entirely up to how you want to read the manga. Like I said, Oda has left some room for flexibility.
--Consistency is a tough one to tackle, and I believe most mangaka fail at it. However, Oda has a great knack at keeping the OP facts consistent. Every arc happens with 2-3 days, and OH SO MUCH HAPPENS EVERYTIME. Like Jobebob said, the routine is the always the same: land on island, discover enemy, fight like crazy, rest/party, goodbye island. If you're concerned about the relationship between time sailed and distance travel, ignore that science crap that Jobebob posted and look at my opinion instead: X) @Kibagami:
if they followed their log post all the way who knows how long it would've taken, but from Little Garden on, they were using an Eternal post to Alabasta, effectively cutting a few islands (except Drum). Moreover, they spent some time in the sky!!! Everyone knows that Sky Island is mobile right? I'm not gonna waste my time proving it to you, but ALL the sky islands are presumably flying around on that giant cloud (can't remember the name). It is impossible to tell how far they flew during their time on Skypiea, or what range the Sky cloud covers, but it's pretty indisputable that they flew in the 'right' direction, and it got them pretty damn close to the Redline. Had they actually sailed to all the islands in that range it may have taken A LOT longer.
Finally does Oda have a timeframe is mind? Is everything part of a deliberate attempt to make the voyage length shorter? I think my answer is obvious.