1. Current solution limit: In general to avoid any complexity on application it's good if component by themselves can show proper availability time slots for the specific time zone. As you already validating selection in dropdown(try to select time which is not available, when we have 23 hours ), your component correcting it to the next proper time slot. 

2. Date Time in specific timezone without magic convert date class into specific timezone one. u use Date class which limits your component on how it can work. We have case when we want to show DateTime picker in specific timezone including proper showing time slots during selection. So in case of transferring date to specific zone you don't know which week for this country is used for date time shift, where you don't know a list of slot (23 hours or 25 hours on this day). You need to build timeslots based on what is available in milliseconds for this day and just format it in dropdown. In this case you will solve both issues.