A light year is not used to measure time. A light year refers to how far light reaches in a year's time. That distance is about 5.87 trillion miles.
Light years are used to simplify the discussion of extreme distances. The distance that light travels in a second is about 186,000 miles, making that the distance of a light second. The star nearest to the Earth is roughly 24 trillion miles, or slightly over 4 light years, away. Light years may also indicate the age of a visible astrophysical event, because it took that much time for the light of that event to reach the Earth.