The rule most people know is incomplete
"Divisible by 4" is the leap-year rule most people learn, and it's correct for the vast majority of years — but it has two exceptions layered on top. The full rule: a year is a leap year if it's divisible by 4, UNLESS it's also divisible by 100, in which case it's only a leap year if it's ALSO divisible by 400. Century years are where the simple "divisible by 4" rule breaks down.
Working through the century-year exception
1900 is divisible by 4 (1900 ÷ 4 = 475) and would be a leap year under the simple rule — but it's also divisible by 100 (a century year), and 1900 ÷ 400 = 4.75, not a whole number, so it fails the "also divisible by 400" test and is NOT a leap year (365 days). 2000 is also divisible by 4 and is a century year, but 2000 ÷ 400 = 5 exactly, so it passes the exception's exception and IS a leap year (366 days). 2100, the next upcoming century year, will follow 1900's pattern — divisible by 4 and by 100, but not by 400 — so 2100 will NOT be a leap year.
Ordinary years follow the simple rule without complication
For any year that isn't a century year (not divisible by 100), the century exception never comes into play at all — the simple "divisible by 4" rule applies directly. 2024 is divisible by 4 and not a century year, so it's a leap year (366 days). 2026 is not divisible by 4, so it's not a leap year (365 days) — this is the overwhelming majority case, which is exactly why the simplified "divisible by 4" version of the rule is the one most people remember, even though it's technically incomplete.
Why the extra rule exists at all
A simple "leap year every 4 years" would slightly overcorrect for how long a solar year actually is (about 365.2422 days, not exactly 365.25) — over centuries, that small excess would drift the calendar out of alignment with the seasons. Removing 3 leap years every 400 years (the century years that aren't divisible by 400) brings the average calendar year much closer to the true solar year length, which is precisely why the Gregorian calendar reform introduced this extra century-year exception in the first place.
Why this matters for date calculations
Getting a leap year wrong shifts every date calculation crossing 29 February by a day — age calculators, days-between-dates calculators, and anything counting days across February in a leap year all depend on this rule being applied correctly, including its century-year exception. A calculator that only checks "divisible by 4" would silently give a wrong answer for any date range crossing a non-leap century year like 1900 or 2100.