gistfile1.py

import datetime

begin = datetime.time(11, 48)
end = datetime.time(12, 15)
#timedelta = end - begin
#print timedelta
timedelta = datetime.timedelta(seconds=26)


def time_add(t, delta):
    dt = datetime.datetime.combine(datetime.date.today(), t) + delta
    return dt.time()

def parse_time(s):
    parts = s.split(':')
    hour = int(parts[0])
    minute = int(parts[1])
    second = int(float(parts[2]))
    microsecond = int(float(parts[2]) * 1000)
    return datetime.time(hour, minute, second, microsecond)

def time_to_str(t):
    return '%d:%02d:%02.2f' % (t.hour, t.minute, t.second)

prefix = 'Dialogue: '
with open('xx.ass', 'rt') as f, open('gg.ass', 'wt') as wf:
    for line in f:
        line = line.decode('cp950')
        line = line.strip()
        if line.startswith(prefix):
            line = line[len(prefix):]
            parts = line.split(',')
            begin = time_add(parse_time(parts[1]), timedelta)
            end = time_add(parse_time(parts[2]), timedelta)
            print parts, begin, end
            parts[1] = time_to_str(begin)
            parts[2] = time_to_str(end)
            print >>wf, prefix.strip(), ','.join(parts).encode('cp950')
        else:
            print >>wf, line.encode('cp950')