At 15:14 of the third period in the Senators/Canadiens game, Ottawa requested a Coach's Challenge to review whether Montreal's Paul Byron was off-side prior to his goal.
After reviewing all available replays and consulting with the linesmen, NHL Hockey Operations staff determined that Byron was off-side prior to the goal. According to Rule 78.7, "The standard for overturning the call in the event of a 'GOAL' call on the ice is that the Linesman, after reviewing any and all available replays and consulting with the Toronto Video Room, determines that one or more Players on the attacking team preceded the puck into the attacking zone prior to the goal being scored and that, as a result, the play should have been stopped for an "Off-side" infraction; where this standard is met, the goal will be disallowed."
Therefore the original call is overturned - no goal Montreal Canadiens.
The clock is re-set to show 4:49 (15:11 elapsed time), when the off-side infraction occurred.
Since the Coach's Challenge resulted in the original call being overturned, the Ottawa Senators retain their time-out.