FDSN beta service waveforms do not begin and end at requested times

This actually holds true for the current fdsn service as well. The below example uses the obspy fdsn client, which appears to pass arguments properly to urls:

from obspy.clients.fdsn import Client
from obspy import UTCDateTime

client = Client('http://beta-service.geonet.org.nz')
st = client.get_waveforms(
    network='NZ', station='CNGZ', location='*', channel='EHZ',
    starttime=UTCDateTime('2019-09-04T04:00:0:0.000000'), 
    endtime=UTCDateTime('2019-09-04T05:00:00.000000'))
print(st)
1 Trace(s) in Stream:
NZ.CNGZ.10.EHZ | 2016-09-04T04:00:01.308441Z - 2016-09-04T04:59:56.868441Z | 100.0 Hz, 359557 samples

While this queries the database accurately (generating the url: http://beta-service.geonet.org.nz/fdsnws/dataselect/1/query?channel=EHZ&station=CNGZ&starttime=2016-09-04T04%3A00%3A00.000000&location=%2A&endtime=2016-09-04T05%3A00%3A00.000000&network=NZ%27

The returned data do not start at the requested start-time, nor end at the requested end-time.

It looks like the FDSN spec states that data can start at the start-time or after (and vice-versa for end-time), but, other services provide data closer to that expected. It would be really nice if the GeoNet FDSN did the same.

I say that this holds for the current FDSN, but that isn't quite true: a similar request using the current FDSN service yields the following data:

1 Trace(s) in Stream:
NZ.CNGZ.10.EHZ | 2016-09-04T03:59:54.568443Z - 2016-09-04T05:00:02.408443Z | 100.0 Hz, 360785 samples

In this case, the data do not appear to meet the FDSN specs, both starting before the given time, and ending after the end-time.

Further to this - a more general question, why do GeoNet data not have samples at zero-millisecond times (e.g. the closest sample to the given start-time is actually at 2016-09-04T03:59:59.998443). Is this an accumulated leap-second thing?

GeoNet / help

FDSN beta service waveforms do not begin and end at requested times #41