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

I have hope that ours will get there one day. Nearly four years in and they’re almost starting to tolerate one another

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.