They most likely modified a ship they already had. And like it was already explained, they have ways to move large amounts of earth and metals that we just don't. Going by our timelines would be kinda ridic. However, even using our timeliness, ships can be built quite quickly during war; during necessary times. WWI & II proved this. Ships, planes, the A bomb, factories, upgraded armaments, production plants, research facilities, testing facilities, etc... all within a small window of time, and with all the country's resources going into their manufacturing, WERE built. A year to modify a carrier? Even for us, that's easily doable and historically has been accompished.