One light-year is 9.4607 trillion km. It is the distance light travels in one year through vacuum — a unit of distance, not time.
Light travels at 299,792.458 km per second in vacuum. That figure is not measured but defined — since 1983 the metre itself is defined by the speed of light.
The word "year" makes it sound temporal, but a light-year is a unit of distance, used where kilometres become unwieldy.
Proxima Centauri is 4.2465 light-years away, Sirius 8.611. The light you see from Sirius tonight left it eight years and seven months ago.
Sunlight takes 8 minutes 19 seconds to reach Earth. If the Sun vanished now, we would not know for 8 minutes 19 seconds.