issue_comments

This logic has been around in xarray for a long time (I think it dates back to https://github.com/pydata/xarray/pull/12!), so it predates me. If I had to guess though, it would have to do with the fact that back then, a form of cftime.date2num was used to encode all times, even those that started as np.datetime64 values. I think that's significant for two reasons: 1. In the old days, date2num would only return floating point values, even if the times could in principle be encoded with integers. For that reason, for accuracy reasons, it was best to keep the encoded values as small as possible to avoid roundoff error. 2. Even if (1) was not the case back then, date2num did not -- and still does not -- support nanosecond units, because it relies on microsecond-precision datetimes.

This of course is not true anymore. We no longer use date2num to encode np.datetime64 values, and we no longer encode dates with floating point values by default (#4045); we use integers for optimal round-tripping accuracy, and are capable of encoding dates with nanosecond units.

To be honest, currently it seems the only remaining advantage to choosing a larger time encoding unit and proximate reference date is that it makes the raw encoded values a little more human-readable. However, encoding dates with units of "nanoseconds since 1970-01-01" is objectively optimal for np.datetime64[ns] values, as it guarantees the maximum range of possible encoded times, and maximum round-trip accuracy, so it could be worth revisiting our approach in light of the fact that it makes appending somewhat dangerous.