When does post season officially begin?

[Impartial Commish]

When does the post season in the NFL officially begin? Does it begin after the last second rolls off the clock in the final regular season game (typically the Monday night game) or does it begin at kickoff of the beginning of the first game of the post season?

I ask because I commission a league and in our rules we state "No post season drop/adds are permitted." This is done for several reasons but primarily to prevent those whom have lost out on the playoffs from dropping players that can be utilized by other teams. I could interpret the rule either way but I want to mirror the NFL as much as possible.

If I have posted in the wrong forum I apologize. This seemed like the most appropriate forum to post.

[Arlo]

I'm not so sure this is clearly defined by the NFL rules - does anybody know?

In any case, I think the best way to ensure that the rule actually serves the intended purpose is to reformulate it (with the league's approval) to state that as soon as a team has been eliminated, regardless of the timing, it may no longer add or drop players.