On the more technical side the days timescale uses DateTime objects to get accurate amount of days per month and it only supports years 1-9999. So if your game on that timescale goes past 9999 it will automatically shift to months scale. Obviously since they go in separate speeds the date will end up completely differently. But the simulation will be fine. Basically it just prevents you from displaying time using the days timescale.
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That’s exactly what I thought. I assumed it was a formatting issue, given that it is a timescale that has been added, effectively a representation of the time that has progressed (and not an actual new speed at which the game advances).
From my understanding, this is then simply a potential aesthetic choice. Nice.