Learn about Leap Year:

One year is not exactly equal to 365 days and 6 hours (365.25). But it is 365.242374 days as per the "Vernal Equinox Year". If you make one day extra for every four year then the next century will be one day in advance than the original.
To adjust that we decrease one day on every century although that is a leap year.

So as per that calculation the actual number of days per year will be 365 + 1/4 - 1/100 + 1/400 = 365.2425

365 - Normal days per year
1/4 - We add one day for every four year
1/100 - We remove one day every century
1/400 - But for every 400 year we add one day

But even this day (365.2425) also 0.000125 days more than the actual days (365.242374) per year.
This means in the year 8000 we need to subtract one day though that is divisible by 4, 100 and 400.
But this may not be necessary as the "vernal equinox year" will have a minor change which may be adjusted.

Vernal equinox year - The sun cross directly over the earth's equator. This time the season will change to spring.
For the year 2008 it is on March 20, 2008 at 1.48 AM EST.