Time Sync Protocol for Lathem Clocks

lathem.py

'''
DATA: 0#112233456677Z,CR (16 chars)
'0' is address byte
'#' is command code for Routine Time Update
where '11' is hex MS,LS of Seconds of Minute (1-59)
where '22' is hex MS,LS of Minutes of Hr (0-59)
where '33' is hex MS,LS of Hour of Day (0-23)
where '4' is hex Day of Week (Sun - 0 etc.)
where '5' is hex Month of Year (1-12)
where '66' is hex MS,LS of Day of Month (1-31)
where '77' is hex MS,LS of Year offset form 1984 (0-127)
where 'Z' is the 1's complement of the mod-64 sum of all 
characters in the string except the CR and the checksum
'''

from datetime import datetime

def lathem_checksum(s):
    sum = 0
    for x in s:
        sum += ord(x)
    return chr((sum & 63) ^ 255)

def to_lathem_time_string(dt):
    #In python week begins on Monday=0
    #Lathem expects Sunday=0
    weekday = (dt.weekday() + 1) % 7
    #Lathem base year is 1984
    year = dt.year - 1984
    
    format = '0#%0.2X%0.2X%0.2X%0.1X%0.1X%0.2X%0.2X'
    buffer = format % (dt.second, dt.minute, dt.hour, weekday, dt.month, dt.day, year)
    return buffer+lathem_checksum(buffer)+'\x0d'

def from_lathem_time_string(s):
    second = int(s[2:4],16)
    minute = int(s[4:6],16)
    hour = int(s[6:8],16)
    month = int(s[9],16)
    day = int(s[10:12],16)
    year = 1984 + int(s[12:14],16)
    return datetime(year, month, day, hour, minute, second, 0)
        
if __name__ == '__main__':
    import sys
    
    now = datetime.today()
    
    if 'write' in sys.argv:
        import serial
        p = serial.Serial(port='COM6')
        p.write(to_lathem_time_string(now))
        p.close()
    elif 'read' in sys.argv:
        import serial
        p = serial.Serial(port='COM6', timeout=30)
        try:
            while True:
                x = p.read(16)
                if x:
                    print repr(x), from_lathem_time_string(x)
        finally:
            p.close()
    else:
        print now, repr(to_lathem_time_string(now)), from_lathem_time_string(lathem_time_string(now))