python 3.6 sum of the short periods between timestamps

I'm doing some work with logs. Need to calculate a sum of time durations when the process was running without long interruptions. Set the maximum possible interruption to 30 seconds. Logs are emitted every 3 seconds.

so, for example if it was running since 10:20:00 (hours) to 10:30:00 and was interrupted from 10:24:10 to 10:27:10, the desired result is the sum of 10:24:10 - 10:20:00 and 10:30:00 - 10:27:10 = 420 (in seconds). However, calculating time difference using datetime types does not provide a valid solution - i suppose it calculates a difference without including a start/end seconds.

here is the solution i came up with (['timestamps'] is a list of datetime timestamps normally emitted every 3 sec):

    for k, v in proc_activity.items():
        proc_activity[k]['duration'] = 0

        start, next = v['timestamps'][0], ''
        for time in v['timestamps']:
            next = time
            diff = next - start

            if diff.seconds < 30:
                proc_activity[k]['duration'] += diff.seconds
            else:
                print("diff: %s" % diff.seconds)

            start = next

        print(f"added: {proc_activity[k]['duration']}")
        diff = v['timestamps'][-1] - v['timestamps'][0]
        print(f"real: {diff.seconds}")

output:

added: 39
real: 45
added: 39
real: 45
diff: 36
added: 155
real: 218

any suggestion how to fix it?

update, sample input data:

{'service_0': {'timestamps': [datetime.datetime(2018, 7, 1, 22, 33, 39, 86170),
                                     datetime.datetime(2018, 7, 1, 22, 33, 42, 33213),
                                     datetime.datetime(2018, 7, 1, 22, 33, 44, 898234),
                                     datetime.datetime(2018, 7, 1, 22, 33, 47, 893731),
                                     datetime.datetime(2018, 7, 1, 22, 33, 50, 928946),
                                     datetime.datetime(2018, 7, 1, 22, 33, 53, 895617),
                                     datetime.datetime(2018, 7, 1, 22, 35, 7, 116182),
                                     datetime.datetime(2018, 7, 1, 22, 35, 10, 105035),
                                     datetime.datetime(2018, 7, 1, 22, 35, 13, 193428),
                                     datetime.datetime(2018, 7, 1, 22, 35, 16, 210135),
                                     datetime.datetime(2018, 7, 1, 22, 35, 19, 168881),
                                     datetime.datetime(2018, 7, 1, 22, 35, 22, 114653),
                                     datetime.datetime(2018, 7, 1, 22, 35, 25, 102365),
                                     datetime.datetime(2018, 7, 1, 22, 35, 43, 46950),
                                     datetime.datetime(2018, 7, 1, 22, 35, 46, 15435),
                                     datetime.datetime(2018, 7, 1, 22, 35, 49, 23333),
                                     datetime.datetime(2018, 7, 1, 22, 35, 52, 22164),
                                     datetime.datetime(2018, 7, 1, 22, 35, 55, 78615),
                                     datetime.datetime(2018, 7, 1, 22, 35, 58, 78573)]}}

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