Select a default time zone only if you are importing from a timezone other than the time zone your Planning Center account is in. You can find your Planning Center localization settings at https://accounts.planningcenteronline.com/organization/localization.
Time zone plays a major role when importing donations that have a contribution date at the beginning of the year, or the end of year. Selecting the wrong time zone may cause donations to import in the wrong year, month, or day. The default option will use your ‘local’ time zone.