A change has been made to the timer rule regarding Pokemon Showdown!, given the recent controversies surrounding it. The timer is to be activated at the first possible opportunity the beginning of each match in accordance with the Timer Clause. Once activated, each player is automatically given 5 minutes, with ten seconds added per turn.
If neither player activates it at the beginning of the match, then the timer may be activated at any point during the match. The timer, when activated, ALWAYS starts at 5 minutes. Thus, if it is activated at a later point in the match, each player will have effectively been given more than 5 minutes to spend on their moves--the time they spent before the timer was activated and the time allotted by the 5 minute timer.
Credit to Luis Negron Photography and McMeghan Animations