2012 was a leap year, 2013 wasn’t, 2016 will be, 2014 won’t be, 2000 was and 1900 was not. Years are about 365 days long, but not exactly (measuring years is kind of hard to do and there are different ways of doing it, but a sidereal year is about 365.256363004 days long, and a tropical year is about 365.24219 days long, the difference is in the way the two are measured), and this difference between clean lengths of time like 365 days and the awkward 365.24219 days is enough to throw seasons off so that summer occurs in December. The Gregorian calendar uses a trick to adjust for this, adding a day to February. Normally, a year in the Gregorian calendar has 365 days, except for every fourth year (4, 8, 12, 16, …2004, 2008, 2012, 2016) which have 366 days. However, every 100th year (100, 200, 300, …2100, 2200, 2300) will not have a leap day, and thus have 365 days. Finally, every 400th year (400, 800, 1200, 1600, 2000, 2400) will have a leap day, and be 366 days long. This brings the average Gregorian calendar year to 365.2425 days long, not terribly different from the 365.24219 day long tropical year. Finally, leap seconds. The length of a day varies slightly, caused by gravitational forces on the Earth by the moon, sun, and other planets and usually only changes by a second, over the course of time. To keep the average length of a day as close to 86400 seconds as possible. Like leap years, leap seconds keep the rigid 86400 second day from drifting so 12-noon was sunset.