There are 31,536,000 seconds in a 365-day Earth year. This number is calculated by multiplying 60 seconds per minute by 60 minutes per hour, and multiplying the answer by 24 hours per day and 365 days per year.
Most Earth years contain 365 days. Leap years, which occur on each year that can be divided evenly by four or 100, contain 366 days. These years are 31,622,400 seconds long. A further refinement to measuring the Earth's revolutions around the sun is the leap second, which is added every two years to maintain consistency. A leap second was most recently added in 2012, which was also a leap year, so there were 31,622,401 seconds in the year 2012.