Take a look at this shot from the third opening of 5D's
Now, I know this pic is kinda small, but take a close look at the signs. They are all words that represent the astrology~ Scorpio, Capricorn, Sagittarius, Gemini, and the Sheep are all visible.
Not only that, but it says 2021 in one of them. Before we get into any hardcore discussions about how this works with DM and GX's timeline, I would like to say that I believe that 5D's is a separate, possibly more advanced universe then DM/GX.
Therefore, seeing that 2021 might possibly represent the year that 5D's takes place and that it said 2021 in a shot in the first opening, which is six months earlier. Its now possible to get some sort of feel for a big scale timeline.
Since seasons 1-3 occur in the same year, and that the timegap between 2 and 3 is six months, this means that seasons 1 and 2 take place sometime in January-June. and that season 3 is July-December. Not much, but its something.
And since Aki and possibly Jack(since we haven't had confirmation about his age pre-timeskip) have had birthdays during the timeskip(they are suppose to be a year older now) this means that their birthdays are sometime between February-June. Yusei's birthday is also during the latter half of the year, since his birthday hasn't occurred yet.
Not much to go on(especially when the 2021 could just be fake), but its a nice start.