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I think Oda said that he intended for his characters to not "age" at all. I don't really understand that, but I'll go with it anyway. He can obscure time quite easily thanks to all the islands having their own seasons.
If we could identify which of the routes they took, we could count the islands between Reverse Mountain and Mariejoa on the map. Chances are that a single island has been skipped, the island of which Robin gave them an eternal pose of after leaving Whiskey Peak. That island was supposedly the stop before Alabasta, although it could have very well been Drum Island (since I don't think the manga declares the island's name).
Isn't the problem basically, that they don't have to stay on one route as soon as they get an eternal pose? with an log pose they have to wait until it points to the next island and so on, but they changed direction for little garden, drum, alabasta and thats the minimum as far as I know.
Other methods of measuring time in One Piece also failed until now like counting the change of cloths or looking at the phases of the moon (that only because we saw a model of the world at Ohara which had multiple moons).