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The Hebrew Calendar: Part 2
Reconciling the Number of Days between the Hebrew and Gregorian Calendars

The Hebrew Calendar (Part 2) 

In part 1 of our series on the Hebrew Calendar we saw how the Hebrew calendar has a different number of days in a year than the Gregorian calendar.  The twelve month Hebrew year has 354 days, eleven or twelve days less than the Gregorian calendar.  In seven years out of a nineteen year cycle, a thirteenth month is added to make up for the lost days.  In the third, sixth, eighth, eleventh, fourteenth, seventeenth and nineteen years out of a nineteen year cycle there are thirteen months. 

            The Hebrew calendar months alternate between 29 and 30 days:

Month Number of Days  
Nisan 30
Iyar 29
Sivan 30
Tamuz 29
Av 30
Elul 29
Tishri 30  
Heshvon 29
Kislev 30
Tevet 29
Shevat 30
Adar 30/29
Adar II 29


             In a twelve month year, the month of Adar has twenty-nine days.  In a leap-year (thirteen month year), Adar has thirty days and Adar II (the added thirteenth month) has 29. So the twelfth month, Adar can have either 29 or 30 years depending on if the year is a leap-year or not.  With that reasoning the Hebrew year can have either 354 or 384 days.  

            Now we need to begin to see if we can see how close we can come to reconciling the number of days between the two calendars.  Although some have had loosely said the number of days matches up over nineteen years, that is not exactly the case, there is a slight difference.  Let's see how close they come. 

  •  A lunar month is actually 29 days, 12 hours, 44 minutes and 2.8 seconds.  This is how long it takes the moon to circle the earth and is what lunar months are based on.  In decimal format this is 29.530587963 days.
  • Twelve lunar months make up 354.37 days.  Twelve months on the Gregorian calendar are 365.2425 days.
  • The number of days in the Gregorian calendar over nineteen year is:           

      19 (years) x 365.2425 (days per year) = 6,939.61 day

  • The number of days in a twelve-month lunar calendar over nineteen years is:

      19 (years) x 29.530588 (days per month) x 12 (months) = 6,732.97 days

  • In the modified lunar (lunisolar) calendar, the Jews insert a thirteenth month (Adar II) in seven years out of every nineteen year cycle.  The additional number of days accounted for by this thirteenth month is:

      7 (years with thirteen months) x (29.53088 (days per month) = 206.71 days.

  • When we add the number of days in nineteen, twelve-month lunar calendar years to the days added during the seven years when there are thirteen months we have:
  •       6,732.97 + 206.71 = 6,939.68 days 

            The Gregorian and modified lunar calendars thus reconcile to within .07 of a day over nineteen years.
6,939.68 – 6,939.71 = .07  

Summary: Number of Days over Nineteen Years

Gregorian (solar) Calendar
Average Days per year =  365.2425
Days in 19 years = 6,939.61

Hebrew (Lunisolar) Calendar
Days per Month = 29.530588
Days per year (12 monrh year) = 354.367056
Days per year (13 month year) = 383.897644
Total days in twelve 12-month years = 12 x 354.37 = 4,252.40
Total days in seven 13-month years = 7 x 383.90 = 2,687.28
Total days in 19 years = 6,939.68

Difference over 19 years = .07 day

             Part 3 of this series will discuss the significance of .07 of a day plus a couple more tid-bits of information. 

For more information about The Hebrew Calendar check out Thomas Thorne's book The Wonder and Majesty of God's Festivals and Holy Days:  The Master Plan Revealed.


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