Converting Gregorian dates to day numbers

def calcday(y, m, d):
    """
    Return days since 2000-01-01
    y: year
    m: month (1=jan)
    d: day (1=first day of month)
    """ 
    #return m*30 + (m+(m>7))//2+d - (m>2)*(2-(y%4==0))+365*y+(y-1)//4 - 730530
    return m*30 + 14*m//25+d - (m>2)*(2-(y%4==0))+365*y+(y-1)//4 - 730530