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.
This is why I got into fountain pens too! These days I find myself using a good rollerball or mechanical pencil for day to day, since they’re a little more practical, but man oh man I do love a fountain pen…