fiddle.py

from time import time

starts = [
    1248637121,
    1248818005,
    1248819043,
    1248819661,
    1248820029,
    1248862668,
    1248976328,
    1248989370,
    1249036736,
]

stops = [
    1248817256,
    1248817974,
    1248819674,
    1248819680,
    1248819813,
    1248819936,
    1248819977,
    1248820035,
    1248820188,
    1248873042,
    1248978031,
    1248992420,
    1249036746,
]

def zip_to_type(starts, stops):
    tuples = []
    for start in starts:
        tuples.append(('start', start))
    for stop in stops:
        tuples.append(('stop', stop))

    tuples.sort(lambda x, y: cmp(x[1],y[1]))

    return tuples

def flush_spurious(tuples):
    good = []
    last_seen = None

    for flag, timestamp in tuples:
        if not last_seen and flag == 'stop': # keep going until we hit a start
            continue
        if flag == last_seen: # Already saw one of these, skip it
            continue
        good.append((flag, timestamp))
        last_seen = flag

    if good[-1][0] != 'stop': # if the last thing is a start, end it with now
        good.append(('stop', time()))

    return good

def totaller(tuples):
    total = 0

    for i in range(len(tuples))[::2]: # take even indices of tuples
        start = tuples[i][1]
        stop = tuples[i+1][1]
        total += (stop - start)

    return total

def count_times(starts, stops):
    return totaller(flush_spurious(zip_to_type(starts, stops)))

if __name__ == '__main__':
    print "with stopper"
    seconds = count_times(starts, stops)
    print float(seconds)/(60*60), 'hours'

    print "without stopper"
    stops.pop()
    seconds = count_times(starts, stops)
    print float(seconds)/(60*60), 'hours'