Captain Kelsey Grammer did it... So is the issue that they can't go forward in time, or that they can't intentionally go forward in time? You'd have to invent the method, but does that depend on where in time you gain the knowledge to do so? If Trek tech was capable of going forward in time say by the 25th century, would it only be possible then... is it all based on when a reliable method is discovered? If you can go back and forth, but go back and let someone else go forward, does that not count? What if another race from another planet was capable of moving forward in time, but did so in Trek's history, maybe about the 20th century, does it count even if Starfleet never knew about it? What about Q?