Time_coords ISO8601 format

(From M. Taylor assessment [2022-01-17])

Sec 4.1:

"... expressed in the ISO8601 format" - ISO8601 gives you rather a lot of options. Do you want to narrow it down a bit?

See e.g. DALI 1.1 sec 3.3.3 (https://www.ivoa.net/documents/DALI/20170517/REC-DALI-1.1.html).

An example would probably do; at least say something about time zones (e.g. is trailing Z required/permitted/forbidden).