Go to workspace settings
In Slack, type the command "/events settings" - you'll see the following buttons:
Click "Change Timezone" to specify a new Timezone for Eventbot. Note this won't change your Slack timezone - it will only affect Eventbot.
Eventbot will assume a default timezone for your workspace based on your Slack workspace settings. To change this select your Timezone region, then choose a Timezone (as shown below).
Changes you should see
Once changed, you should this take effect immediately. Changing the timezone only changes how events are input. If you type "/events create" you should see the default timezone has been updated.