It’s March 1 this year. And the time is important. Officially all trades must be completed by 3 p.m. New York time. What usually happens is that the “trade calls” you hear about on the deadline shows get backed up and there may be trades announced after that time.
In truth, teams can make any trade they like after the deadline, and it does sometimes happen. The deadline is really for playoff eligibility in the NHL. You must be a on the roster of the NHL team at the deadline to play in the playoffs for that team.