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because earth takes 365 days and 6hours to complete its revolution and 1st,2nd,3rd year the 6hours is not counted in these first 3 year and on fourth year the 6 hours of first year ,second year,third year's 6 hours is counted on fourth year As=====> 6hours + 6hours + 6hoursand fourth's year 6hours are also plus together=24hours so 24hours = 1 day so on 4th year365+1day =366days as become leap year. so february becomes 29