begin;
update contact set timezone = company.timezone from company where contact.company = company.id and company.timezone != '' and contact.timezone = 'UTC';
-- above, we update all users with UTC as tz to their company's tz, if set. 
-- below, we update all user with UTC as tz to the system tz value.
update contact set timezone = system_get_timezone() where contact.timezone = 'UTC';
commit;
