These Ruling(s) have not been confirmed as applicable to the TCG.[Notes 2]
For End of Match procedures, you resolve all appropriate effects before checking Life Point difference. For example, if the final turn of End of Match procedures is also the last turn for the effect of "Final Countdown", the effect of "Final Countdown" would be applied before the End of Match procedures Life Point check.[Notes 1]
Previously Official Rulings
These TCG rulings were issued by Upper Deck Entertainment and have since been deemed unofficial by Konami. They were previously considered official and can still be correct, as long as they follow the official gameplay rules, or unless Konami has issued a ruling that says otherwise. References: 
You count turns for "Final Countdown" like this: Turn 1 is the turn you activate it, Turn 2 is your opponent's turn, Turn 3 is your turn after you activated "Final Countdown", etc.
You can activate "Pyro Clock of Destiny" to advance the turn count by 1 for Final Countdown. Final Countdown does not have to be on the field when you use Pyro Clock of Destiny.
You win when the 20th turn ends, before the Draw Phase of the 21st turn.
When your opponent chains "Imperial Order" to your Final Countdown, the effect of Final Countdown is negated and you cannot start counting turns. But if Final Countdown is activated and resolves, and later Imperial Order is activated, it will have no effect on Final Countdown because it will be in the Graveyard.
When you activate Final Countdown, and your opponent activates Final Countdown during his next turn, and then uses Pyro Clock of Destiny to advance his/her Final Countdown by 1, both Final Countdowns win at the same time and the Duel is a Draw.
↑"End of Match" is a procedure that is applied in official Tournaments when a Duel takes a long time to finish. The Judge announces to the Duelists that the Duel will end in a specific ammount of turns and when the final turn of End of Match procedures finish, whichever Duelist has the most Life Points is declared the winner.
↑Konami Judge Program Rulings are considered unverified, as their source(s) are not publicly viewable. They are not confirmed as applicable to the TCG until they have been officially published by Konami.