HamStudy is the best one. The app cost a couple bucks but the mobile site is free

Sometimes I really dislike being in Columbia, but seeing views like this make me take a step back and appreciate it for what it is

A slightly easier way to think of this: leap years happen every 4 years, except that we skip one every 100 years (those that end in 00) unless it’s divisible by 400. So we had a leap year in 2000 because it’s divisible by 400, but 2100 will not be a leap year.