The cutoff date is always the Jewish holiday of Passover, which falls during the springtime. Basically, grain that takes root before Passover is considered Yoshon. Winter wheat is always Yoshon because it was planted before Passover. The problems with the supply of Yoshon start when the new season's spring wheat is distributed. This date varies from year to year.
Spring wheat is preferred in cake flour, so it is more problematic.
Practically speaking, rye flour is not an issue because it is a winter crop only. The two grains of most concern are wheat and oats.