Answer:
You need to account for the leap years. It's done like that:
A leap year is every year divisible by 4 (unless it's divisible by 100 (unless it's divisible by 400)).
365 + 1/4 − 1/100 + 1/400 = 365.2425 days in one year on average
Sometimes people think the answer is 365.25. This is incorrect and assumes that every 4th year is a leap year, while in fact it's not. Actually it's approximately every one year in 4,12 years.