Why will 2100 not be a leap year?

A year is longer than 365 days so the need to add a leap year every 4 years. But it is slightly less than 365.25 days so you don't want every fourth year to be a leap year. The current solution is that every fourth year is a leap year except when it is a century (a year ending in 00). This is a leap year only if it is divisible by 400 and not otherwise. So 2100, a century but not divisible by 400, is not a leap year.