Your account timezone is the timezone you see on your main dashboard in the bottom left corner.
Note that we pull this data from your Apple Search Ads account. In Apple Search Ads you can select your time zone when you create your Apple Search Ads account. This selection defines start times and end times for all your ad groups and reporting metrics.
Once you set the account time zone for your account, you cannot change it. In the same way SearchAdsHQ pulls the data you have specified, and this can't be changed on our side either.