[Burt the Butcher]

It's a little hard to find in the rules...we really should add it under Key Dates section also - but here it is:

The deadline for the announcement of the length of the contracts of players picked up in free agency or the rookie draft will be August 15th each year by midnight (the night before the start of the In-Season Free Agency period which begins on August 16 each NFL season). No player can be traded unless his contract years are declared BEFORE the trade is announced.