Steps to reproduce

1. Start up a Cloud instance with the timezone set to Sydney time (GMT+11)
2. Set your profile's time zone within Confluence to be Hawaii (GMT-10)
3. Create a task on a page, assigned to yourself, and with a due date of the current date as of Hawaii time
4. Complete the task and look at the "Completed on" date in Profile > Tasks.

Expected result

The "Completed on" date is today as of Hawaii time

Actual result

The "Completed on" date is listed as what would be tomorrow in Hawaii time (today in Sydney time, i.e. server time).