start = time.mktime(t[:3] + start + t[6:])
    finish = time.mktime(t[:3] + finish + t[6:])

import time
def func(start, finish):
    t = time.localtime()
    start = time.mktime(t[:3] + start + t[6:]) # подменяем ч:м:с в полученном кортеже time.localtime() своими
    finish = time.mktime(t[:3] + finish + t[6:]) # аналогично.
    if start >= finish:
        finish += 86400
    return finish - start
start = (12, 0, 0) # часы, мин, сек.
finish = (20, 30, 0) # часы, мин, сек.
print func(start, finish)

>>> import time
>>> tlkt = time.localtime()
>>> tlkt
(2012, 12, 4, 21, 8, 37, 1, 339, 0)
>>> tnew = tlkt[:3] + (22, 0, 0) + tlkt[6:] # заменили время на свое.
>>> tnew
(2012, 12, 4, 22, 0, 0, 1, 339, 0)
>>> time.mktime(tnew) - time.mktime(tlkt)
3083.0 # разница между 22:00:00 и 21:08:37
>>> 

start = [20, 30, 0]
finish = [19, 0, 0]
start =  start[0]*3600 + start[1]*60 + start[2]
finish = finish[0]*3600 + finish[1]*60 + finish[2]
if finish <= start:
    finish += 86400
print finish - start