When does a down begin
 

Does a down begin at the ready or when it is snapped?, I could not find clear answer in rule book.

Also the background to this question is the whole New Mexico mess where the refs started the clock after a penalty. I saw this argument but I'm not convinced

: Rule 3-3-3 -Me: I was. ... 3-3-3 A period must be extended by an untimed down if during the last time down of the period there was a foul by either team and the penalty is accepted

the argument made was that since there was a penalty on the last play of the game the game should have been at least extended one untimed down.

However, if a down starts at the ready and no at the snap then there was no penalty during the last time down and therefore the situation was handled correctly.

Down starts by snap or free kick.

2-7-1 A down is action which starts with a legal snap or when the ball is kicked on a free kick.

The try starts on the ready-for-play per 8-3-2.
For penalty enforcement, the ready-for-play will determine how a penalty will be enforced and what the down would be for a dead ball foul.