If you need to update your clinic's timezone, you can do this from Business Setup -> Your Clinic -> Clinic Tab -> Clinic Information -> Timezone. This will set your diary and online booking timezone for your Clinic.
Click on the Save button at the bottom of the Business Setup page to save the changes.
Please note, this is the Timezone set your clinic as a whole. If you see clients internationally or interstate, we do offer multiple Timezone support. Instructions for this can be found here: How to enable multiple timezone support