Ocean output Time and xtime discrepancy

[elynnwu]

@golaz had a v3.LR.piControl run with custom ocean output that should be 5-daily:

<stream name="timeSeriesStatsCustomOutput"
        type="output"
        precision="single"
        io_type="pnetcdf"
        useMissingValMask="true"
        filename_template="v3.LR.piControl.mpaso.hist.am.timeSeriesStatsCustom.$Y-$M-$D.nc"
        filename_interval="00-01-00_00:00:00"
        reference_time="01-01-01_00:00:00"
        output_interval="00-00-05_00:00:00"
        clobber_mode="truncate"
        packages="timeSeriesStatsCustomAMPKG">

    <var name="xtime"/>
    <var name="ssh"/>
...

We're seeing a discrepancy in Time vs. xtime. All output variables use Time as the dimension, for example:

	float timeCustom_avg_ssh(Time, nCells) ;
		timeCustom_avg_ssh:units = "m" ;
		timeCustom_avg_ssh:long_name = "sea surface height" ; 

but Time has non-uniform output:

Time = 173861.5, 173867.5, 173873.5, 173879.5, 173882, 173888 ;

or in xarray cftime:

array([cftime.DatetimeNoLeap(477, 5, 2, 12, 22, 30, 0, has_year_zero=True),
       cftime.DatetimeNoLeap(477, 5, 8, 12, 22, 30, 0, has_year_zero=True),
       cftime.DatetimeNoLeap(477, 5, 14, 12, 22, 30, 0, has_year_zero=True),
       cftime.DatetimeNoLeap(477, 5, 20, 12, 22, 30, 0, has_year_zero=True),
       cftime.DatetimeNoLeap(477, 5, 23, 0, 22, 30, 0, has_year_zero=True),
       cftime.DatetimeNoLeap(477, 5, 29, 0, 22, 30, 0, has_year_zero=True)]

However, the corresponding xtime is indeed 5-daily as specified:

 xtime =
  "0477-05-06_00:00:00",
  "0477-05-11_00:00:00",
  "0477-05-16_00:00:00",
  "0477-05-21_00:00:00",
  "0477-05-26_00:00:00",
  "0477-05-31_00:00:00" ;

though the xtime start/endCustom is questionable:

 xtime_startCustom =
  "0477-04-29_00:30:00",
  "0477-05-06_00:30:00",
  "0477-05-13_00:30:00",
  "0477-05-20_00:30:00",
  "0477-05-20_00:30:00",
  "0477-05-27_00:30:00" ;

 xtime_endCustom =
  "0477-05-06_00:00:00",
  "0477-05-11_00:00:00",
  "0477-05-16_00:00:00",
  "0477-05-21_00:00:00",
  "0477-05-26_00:00:00",
  "0477-05-31_00:00:00" ;

We assume xtime to be correct and represents the end of the interval, but we're wondering what exactly does Time represent and that perhaps all variables should have xtime as its dimension.