Do We Really Know How Many Days there are in a Year on the Gregorian calendar?
The method of reckoning time used by most of the world today is known as the Gregorian calendar. It was sponsored by Pope Gregory XIII in 1582 as a corrected version of the Julian calendar. It is a solar based calendar in which dates represent the position of the earth in its revolution around the sun. Before we delve into the Hebrew calendar we need to make something clear about the Gregorian calendar.
The title to this article seems to pose a pretty palpable question; but let's see if we really do know how many days, on average, there are in a year. In the Gregorian calendar there are 365 days in a normal year. Leap years, years with 366 days occur in years that are exactly divisible by four with the following exception: centurial years that are exactly divisible by 100 and not by 400 are not leap years.
To help us understand, let's look at some examples. The year 2000 was a leap year, as was 2004 and 2008. 2012 will also be a leap year. They are four years apart and exactly divisible by four. Now let's examine the time span from 1600 to 1999, which encompassed four hundred years. And let's focus on the centurial years (1600, 1700, 1800 and 1900). The year 1600 was a leap year because it is exactly divisible by one hundred and four hundred. Although the years 1700, 1800 and 1900 are centurial years exactly divisible by one hundred, they are not divisible by four hundred. Therefore, they were not leap years. 

If leap years occurred every four years without exception, we would expect to see twentyfive leap years over one hundred years and one hundred leap years over four hundred years. But, over a four hundred year period, three of the years that would appear to be leap years are not. Even though they are exactly divisible by four they are centurial years not divisible by four hundred. Therefore, there are actually only ninetyseven leap years in a four hundred year cycle. Now we can calculate the average number of days in a year when using the Gregorian calendar as follows:

There are 303 "normal" years in a four hundred year period and 365 days in a "normal" year. 303 (years) x 365 (days per year) = 110,595 days

There are 97 "leap" years in a four hundred year period and 366 days in a "leap" year. 97 (years) x 366 (days per year) = 35,502 days

The total number of days over a four hundred year cycle is 110,595 + 35,502 = 146,097

The average number of days in a year on the Gregorian calendar is 146,097 (days) / 400 (years) = 365.2425 days/year.
In the Gregorian calendar which is used by most of the world, there is an average of 365.2425 days in a year.
Chapter 12 of the book The Wonder and Majesty of God's Holy Days provides a comparison between the Hebrew and Gregorian calendars.
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