It would be useful to provide a version of %datetime where an iana time zone could be passed instead of a numeric offset from GMT to return date and time.

I have a requirement to determine the date and time in another timezone.   This includes timezones in MST (Arizona/Phoenix) which does not observe DST.

If I am in Arizona/phoenix, %timezone will always return DST flag as false even when it applies in other parts of the US.   So if I need to convert the system time in MST to say EST, I can't work out what offset to add as I don't know if EST is in DST or not.
 
If it is the other way round, ie, I am in EST and need to work out what the date and time is in MST then I can determine if EST is in DST and adjust the offset.

However, this does not work if I am not in the US, as US DST has different start and end dates than UK and I assume rest of the world.

All of this could be solved if I could just call %datetime and pass the iana timezone name I need