I’m in Perth, Western Australia and I was adjusting my availability for today. While it was before 10:45 (GMT+8) in Perth, my YouCanBook.me was showing that someone could only book time for 12:45 as the earliest. There were no calendar entries to eliminate any booking periods from 10:45 to 12:45. As I refreshed the page and the clock ticked to 10:45 I found that my next 15 mins, i.e. 12:45 to 13:00 was greyed out.
I believe it is mapping Australia/Perth (which is GMT/UTC+8) to the eastern states of Australia (i.e. Melbourne, Sydney, Hobart) which are GMT+10.
I tried changing to another timezone and then back to Australia/Perth but it made no difference.
PS Choosing Melbourne or Victoria I found that they are also out. For example, Melbourne is now 14:03 (GMT+11 (daylight saving time now)) and the calendar is showing 16:15 as the next available instead of 14:15.