Hi @Lumen5KJ ,
Yes, you cannot set custom date ranges for availability for a different time zone while currently being in the different time zone. Only work-around I see is creating a different meeting link with different time zone and share with your clients.
If you stromgly feel about this idea, I would recommend you to put the same in the idea section of the community. HS product team review these ideas and work on them.
Hope this helps!
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Thanks and Regards.