How to change time zone on Samsung Galaxy Tab?

I recently bought the Samsung Galaxy Tab in Hong Kong. I do not live in Hong Kong so I would like to change the time zone back to where I live. Unfortunately, after I select the time zone I want and i turn off the tab, the timezone automatically goes back to the Hong Kong timezone! I am aware that there is an automatic button but my tab does not have this button. How can i change my time zone? Thanks for your help!!!!