day_of_week.py

def day_of_week(date):
    """ get weekday from date (as instance 2022-12-31 iso format) """
    # 
    year = int(date[0:4])
    month = int(date[5:7])
    day = int(date[8:10])
    offset = [0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334]
    week   = ['Sunday', 
              'Monday', 
              'Tuesday', 
              'Wednesday', 
              'Thursday',  
              'Friday', 
              'Saturday']
    after_feb = 1
    if month > 2: after_feb = 0
    aux = year - 1700 - after_feb
    day_of_week  = 5
    day_of_week += (aux + after_feb) * 365                     
    day_of_week += aux / 4 - aux / 100 + (aux + 100) / 400     
    day_of_week += offset[month - 1] + (day - 1)               
    day_of_week %= 7
    return day_of_week

julian.py

def julian(y,m,d):
    a = (14 - m)//12
    y += 4800 - a
    m += 12*a - 3
    return (d + ((153*m + 2)//5) + 365*y + y//4 - y//100 + y//400 - 32045) % 7