@rayleigh92:
There's still the problem of the Sunny, completed in late march, making Summit War saga happens in the early/middle may and Luffy's tattoo in early June. Even if we don't take the 2Y-day in account, there's still the inevitable fact two years passed, so the reunion is still around may-july. I think it's safe to say that late january can't be the date as it'd make the timeskip last only one year and half (and we got a good amount of facts denining it, beginning with Luffy claiming Rayleigh left Rusukaina half year ago, after Luffy learned all the basics in one year and half).
@Vongola_Boss_XI:
Right, so on that basis alone, we have to assume two months have passed since Zou unless we say Zou wasn't really a full moon (it appears full for 2 - 3 nights, though the trip from Zou to WCI is said to take a good amount of time, to the point the crew were starving, so I'm not sure this works).
But a full moon between Wano and WCI seems pretty set in stone.
I'm telling you people, the lunar cycle is 16 - 17 days!
For real, we have a pretty good sense that the newspaper distribution in-universe is quick enough to get most things out next day. The paper reports on the attack on Whole Cake the day the Reverie starts, and reports on the start of the Reverie the day Luffy arrives in Wano, so it's only two days' travel between the two, and then you get the two week wait for the full moon at the Fire Festival.
Now, there is a possible discrepancy in Luffy and Morgans only escaping Whole Cake after midnight, in the small hours of the morning. It's hard to say if Morgans would have been in time to put the report in that day's edition. We know from Dressrosa that they're able to do special afternoon issues as well as the normal morning ones, but it seems to be a rarity. So it's hard to say if the events of chapter 903 happened on the same calendar day they left Whole Cake, later in the afternoon, or if they're happening alongside the morning edition of the next day after that. So the cycle is 16 or 17 days, depending on which of those is true.
But also no lunar cycle, 16 days or 30, makes the moon phases fit with the dates and travel times pre-timeskip. It kinda works if you look at only the moon, not counting the half moon and full moon on consecutive nights on Skypiea, but otherwise the phases go forward as you'd expect. But they don't work with the Grand Line entry and Sunny completion dates. Basically you get one or the other, not both.
@rayleigh92:
Ten days apart is not exactly like reunite half year before and still claim two years passed. I'll try to schemize it to make it clearer (I'm not a native english speaker, I'm afraid I'm not explaining myself right).
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25 march, the Sunny is fully built and the crew leaves Water Seven
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an undefinied time later Thriller Bark happened. Not too much: Kuma inform Moriah BB joined Warlords here, so not too much after Banaro Incident (which happened the 21 march)
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four days later SHPs leave Thriller Bark
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a few days later SHPs reached the Red Line. The very same day Kuma disperses all of them along the world.
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three days later Luffy reaches Amazon Lily and in the night sail for Impel Down. This is also the original appointment with Rayleigh after coating the Sunny
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five days later Luffy intrudes in Impel Down and all sort of shit happen. Luffy got poisoned and begin the Hormon cure
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26h later Ace is deported and Luffy begins his evasion. Summit War happens.
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two weeks later Luffy wakes up from Marineford's injuries
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an undefinied time later the Ox Bell Incident happens and Luffy show his brand new (temporary) tattoo on the newspaper
We can surely claim that between the Red Line and Luffy's awakening after Marineford 23-24 days passed. As Thriller Bark is surely not so much far from W7 and the Red Line, I'd say this'd make the final date around the early may. This can be stretched only between Thriller Bark and Red Line ("several days") and between Luffy's awakening and Ox Bell (unknown), but that much. If both are alike two weeks, we'd reach early june.
Let's say Luffy in june shows his tattoo: the SHPs will reunite "around two years after this", or at least we are sure Luffy left Rusukaina two years later (he claims Rayleigh left the island half year before, as Luffy learned all haki basics in one year and half, not so much to argue about it). And that very day they reunited at Sabaody and the post timeskip adventure begin.
It's not a matter of "they met the exact day two years later", but that two years actually passed, not one and half or two and half. Two years. Even taking in account a range, it can't be more than a month of difference.
I think the early May date for start of the timeskip is accurate. Water Seven is supposed to be pretty close to the Redline, so I wouldn't want to stretch the post Thriller Bark travel time up two weeks each. It wouldn't feel consistent with how quickly the crew tends to get from place to place otherwise.