average time calculation??

[pmec]

Hi there guys i've got a script that's suppose to find the average of two times as strings. The times are in minutes:seconds:milliseconds
i'm doing ok in printing the right minutes and seconds my problem is with the milliseconds.

Example if i have 00:02:20 and 00:04:40 the average will be 00:03:30 or 00:02:00 and 00:03:00 will be 00:02:30

Can anyone help me out with this please. Here is the code that i have so far:

def lap_average(lap1, lap2): 

    t1 = lap1.replace(":",'') 
    t2 = lap2.replace(":",'')   

    mins1, secs1, hundreths1 = t1[:2], t1[2:4], t1[4:] 
    mins2, secs2, hundreths2 = t2[:2], t2[2:4], t2[4:] 

    total_seconds = int(secs1) + int(secs2) + int(mins1) * 60 + int(mins2) * 60 

    millisec = (total_seconds * 1000)  
    millisec = millisec / 2 

    micro_x = millisec 
    minutes = micro_x / (60*1000) 

    micro_x = micro_x - minutes * (60*1000) 
    seconds = micro_x / 1000 
    micro_x = micro_x - seconds  

   print '%02d:%02d:%s' % (minutes, seconds, micro_x) 

lap_average('03:40:00', '05:20:00') 
lap_average('03:00:02', '02:00:00') 
lap_average('02:25:50', '06:50:75') 
lap_average('00:02:00', '00:03:00') 
lap_average('00:02:20', '00:04:40') 
lap_average('02:40:40', '03:30:30') 
lap_average('02:60:30', '60:40:40')

Thanks in Advance