When you signed up you selected your timezone, but it's worth checking.
Go to Settings
In the Timezone setting, select the correct Timezone option from the dropdown
Once set appointment reminders will be delivered according to your appointment reminder settings.
Your Better HQ clock will also be set to the correct time.
Set different time zones for each team member
Go to Settings > Team > Team Members
Select the Team Member or select 'Add Team Member'
In the personal details box go to 'Timezone'
Select the team member timezone from the dropdown menu