It would take one year for them

[tufguy]

"He told them that it would take one Year for them to reach the Earth".

Please check.

[Rover_KE]

There's no reason to capitalise 'year'.

In what context do you want to use this sentence?

[Charlie Bernstein]

Or: He said it would take a year to reach Earth.

That's ten words instead of fifteen.

[tufguy]

There's no reason to capitalise 'year'.

In what context do you want to use this sentence?

Have you seen Dragon Ball Z?

[Charlie Bernstein]

No. So I just Googled it and learned that it's a movie.

So . . . in what context do you want to use this sentence?

[tufguy]

No, mainly its a cartoon series (there are movies as well) its the World's number One animated action series. In one episode Goku (patragonist) is told by an alien that two aliens were coming to invade the Earth. They were on their way and it would take One year for them to reach the Earth.

[Matthew Wai]

An alien told Goku that two invaders would reach the Earth after one year.

[Piscean]

They were on their way and it would take One year for them to reach the Earth.

There is no need for an initial capital letter for 'one', or for the definite article before 'Earth'. Otherwise it's OK.