WMG / Safety Not Guaranteed

They're successful

They go back to the past and return to the two agents and Jeff & Arnau on the shore. Because government witnesses and journalists are there, the machine's existence is made public. Kenneth is cleared on his charges and gets the patent. Everyone lives happily ever after.

They altered the timeline

Which is why they never came back immediately after leaving. They went into an alternate timeline and thus came back to present in said timeline.

The movie is just a Pilot Movie for...

...a show called TIME BOAT. Kenneth and Darius are stuck in a Sliders kind of concept. (yes, I know it's not really but admit it, it SOUNDS like a TV Show!)

The time machine runs hot and needs a heat sink

That's why it's built into a boat - there's a heat sink underneath to transfer the extra heat to the water.

Jeff will be fired when he gets back

He has no story at all. He wasn't shown documenting anything and it's going to be difficult to get any proof of what happened. He'll probably just look like a madman.

Kenneth doesn't place the wanted ad until after he and Darius go back in time.
He doesn't seem to remember the ad when he first meets Darius. The ad says that he has traveled back in time before, but Kenneth is still collecting parts with which to build his machine. Before leaving, he states that he is now going back "for her".

The Time Machine never worked.

At the end, we just saw it jump out, but whose to say it didn't immediately atomize itself, and end the story right there?

The "Once before"

The first time Kenneth went back in time he did it alone. He saved Belinda but then, realizing than she was never interested in him, ended up crashing the car in her living room. The time travel messed up his memories. That's why he took a moment to react when Darius tells him she spoke with Belinda—one part of him remembers the timeline where she dies and another the one where she lives.