The speed of light is approximately 299,792,458 meters per second (186,282 miles per second). To calculate the time it takes for light to travel one kilometer, we can use the following formula:

Time = Distance / Speed

Time = 1 km / 299,792,458 m/s

Time ≈ 3.33564 * 10^-9 seconds

Therefore, it takes approximately 3.33564 nanoseconds for light to travel one kilometer.