time units in CORDEX-CMIP6 archiving specs

[jesusff]

The specs currently state that:

The unit of the time coordinate is days since 1950-01-01T00:00:00Z or days since 1950-01-01 for all files

This unit formatting as a timestamp (instead of e.g. days since 1950-01-01 00:00:00) is inherited from the previous CORDEX (-CMIP5) specifications.

While CF delegates the interpretation of the time units to the udunits2 library, the examples given in the CF conventions for these units use the days since DATE CLOCK formatting. Also, I couldn't find any file from CMIP6 or CORDEX(-CMIP5) formatted using the days since TIMESTAMP format suggested in the archiving specifications. I wonder whether the CMOR library is automatically writing the time units in a DATE CLOCK format.

So, even though udunits2 seems to accept the timestamp formatting, we might consider updating the CORDEX-CMIP6 archive specs to the actual use of these units in the output files.

ping @gnikulin @larsbarring

[jesusff]

In CMIP6, they just skip the time of the day in the time units examples, e.g. days since 1950-1-1 https://docs.google.com/document/d/1os9rZ11U0ajY7F8FWtgU4B49KcB59aFlBVGfLC4ahXs/edit#bookmark=id.sv0qr44ynpy4