Particularly noticeable if you start in the southern hemisphere, or near a pole.

When loading new chunks in a world that has yet to see the first precipitation since the calendar start (i.e. on June 1 or at a similar time), no snow is placed, even in areas that are permanently snowy (poles) or should be snowy at that time (temperate southern hemisphere).

The same is true for ice, but less noticeable because ice freezing is somewhat slow anyways.

This could be fixed by simulating weather for negative calendar ticks, or by a special case for chunks loaded near the calendar time of world creation.