path: celery/util/time.py
When relative is set to True, the day, hour, minutes second will be round to the nearest one, however, the original program do not update the microsecond (reset it). As a result, the run-time offset on the microsecond will then be accumulated.

For example, given the interval is 15s and relative is set to True

1. 2018-11-27T15:01:30.123236+08:00
2. 2018-11-27T15:01:45.372687+08:00
3. 2018-11-27T15:02:00.712601+08:00
4. 2018-11-27T15:02:15.987720+08:00
5. 2018-11-27T15:02:31.023670+08:00