To make it easier to visualize we'll start with year 400. From year 400 to year 799 you've got a leap year every 4 years except for years 500, 600, 700 and including year 400, so that's 25 leap years for the first century and 24 for the others.
So you've got 25 + (3x24) = 97 leap years
And 75 + (3 x 76) = 303 non leap years
(97 x 366) + (303 x 365) = 146097 days every 400 years which means a year is 365.2425 days long on average.
365.2425 x 24 = 8765.82 hours on average
8765.82 x 60 x 60 = 31 556 952 seconds per year on average
31 556 952 x 299 792 458 (speed of light per second) = a light year is 9460536207068016 meters long or 9460536207068.016 km long when adjusted to take leap years into consideration.