Calculates the number of days from the day on which the Gregorian Calendar starts.

calc_gregorian_long_date.rb

# Calculates the number of days from the day on which
# the Gregorian Calendar starts -- the zero day, Octorber 15th, 1582.
def calc_gregorian_long_date(day, month, year)

    # Note:
    # This method provides no parameter validation.
    # Parameters must be AFTER the day on which the Gregorian Calendar starts
    # -- the zero day, Octorber 15th, 1582.

    zero_date = {"day"=>15, "month"=>10, "year"=>1582}
    days_of_month = [0,31,28,31,30,31,30,31,31,30,31,30,31]
    
    days_by_years = 0
    for i in zero_date["year"]..(year-1)
        if(((i%4==0) and (i%100!=0)) or (i%400==0))
            days_by_years += 366
        else
            days_by_years += 365
        end
    end
            
    days_in_the_year = day
    for i in 1..(month-1)
        days_in_the_year += days_of_month[i]
        if(i==2 and (((year%4==0) and (year%100!=0)) or (year%400==0)))
            days_in_the_year += 1
        end
    end
    
    days_in_zero_year = zero_date["day"]
    for i in 1..(zero_date["month"]-1)
        days_in_zero_year += days_of_month[i]
        if(i==2 and (((zero_date["year"]%4==0) and (zero_date["year"]%100!=0)) \
                       or (zero_date["year"]%400==0)))
            days_in_zero_year += 1
        end
    end
            
    return days_by_years + days_in_the_year - days_in_zero_year
            
end

# test
puts calc_gregorian_long_date(21,12,2012)