2016 is a leap year because it is a multiple of 4 (and not a multiple of 100). The Gregorian Calendar system of leap years gives an average tropical year of 365.2425 days compared to the current true value of 365.2421891, but I think we can do better. Yes, at the 1:20-ish point it should be "365.2421891 days" as is on the screen, not the "365.2521891 years" I say. Sorry, it's been a busy week and I recored and edited the video a bit too quickly to spot Past Matt's mistakes. Read Adam Goucher's article here: https://cp4space.wordpress.com/2012/09/12/lunisolar-calendars/ Music by Howard Carter Design by Simon Wright MATT PARKER: Stand-up Mathematician Website: http://standupmaths.com/ Book: http://makeanddo4D.com/ Nerdy maths toys: http://mathsgear.co.uk/