# Tropical year is 27s/year off from Gregorian year?

http://hpiers.obspm.fr/eop-pc/models/constants.html states the tropical year is $365.242190402$ days.

The Gregorian calendar's average year is $365.2425$ days (every 4th year a leap year, except every 100 years, except every 400 years).

The difference is $0.000309598$ days, which amounts to $26.7493$ seconds.

This is much more than the 1 leap second we add every 2-3 years or so.

What am I missing?

What you're "missing" is that you're basically right! The Gregorian calendar is slowly losing some accuracy. I've previously heard the error quoted as about 3300 yr, which lines up with your measurement because 1 day / 26.75s $\approx$ 3230.