Z suffix (and general time zones) in ISO 8601 timestamps

Super useful workaround: https://github.com/astropy/astropy/issues/8720#issuecomment-717599497

In https://github.com/astropy/astropy/issues/2203 we discussed allowing the Z suffix in ISO strings, which indicates that the time is in UTC, and that got resolved. However, there was a discussion in there about outputting the Z suffix when formatting times, which was not conclusive.

I’ve now run into a related problem while developing time series-related functionality - specifically, if I do:

In [3]: t = Time('2010-01-01 00:00:00', scale='utc')                                                                                                   

In [4]: t.isot                                                                                                                                          
Out[4]: '2010-01-01T00:00:00.000'

then pass the resulting string to other tools, if they conform to ISO 8601 they will interpret this string as a local time, not a UTC time. From ISO 8601:

The zone designator is empty if use is made of local time in accordance with 4.2.2.2 through 4.2.2.4, it is the UTC designator [Z] if use is made of UTC of day in accordance with 4.2.4 and it is the difference-component if use is made of local time and the difference from UTC in accordance with 4.2.5.2.

Technically speaking, the outputted time above is wrong because I explicitly stated it was in UTC and yet the format doesn’t include Z.

At the moment, I’m finding myself having to create a whole new array of times with the Z suffix, and it would be much nicer if we could include the time zone information by default. Even if we don’t have full support for arbitrary time zones, I feel like if the time is a UTC time we should be including Z as a special case.

[Cadair]

Always follow the standards 😉

[mhvk]

I think that we can, for now, just follow what we do for datetime, where we interpret timezones and allow them to be set on output, but do not store it in the Time instance. See http://docs.astropy.org/en/latest/time/index.html#timezones