A distance defined using light speed
A Julian year contains 365.25 days of 86,400 seconds, or 31,557,600 seconds. Multiplication by vacuum light speed, 299,792,458 metres per second, gives approximately 9.461×10¹⁵ metres.
The convention does not change between leap and non-leap calendar years. It also uses vacuum propagation, rather than light travelling through air, water or another medium.
Distance and the age of received light
In a simple nearby-distance model where expansion can be neglected, light from ten light-years away takes about ten years to arrive. Observers consequently receive earlier emission rather than an instantaneous report of a distant object.
Very distant cosmological sources require a specified distance measure and expansion model. Every distance expressed in light-years cannot automatically be read as an identical numerical lookback time.
Conversions and actual journeys
One pc is about 3.26 light-years and one light-year about 63,241 au. Astronomers choose convenient numerical scales without changing the underlying kind of quantity.
A spacecraft moving far below light speed takes much longer than one year to cross a light-year. Its journey also depends on speed changes and route, none of which are specified by the distance unit.