Redmine

How dose Redmine determine the current date?

The problem is:
A Redmine(V2.6) application on Openshift, where the time zone is "-5:00".
The time zone of "My Account" is "+8:00".

On the "Issues" page of project, the "Updated" time is shown as "+8:00", and this is exactly what I expected.

But in the "New issue" page, the "Start date" is shown as "-5:00".
The "this day" shown in "Calendar" page is wrong, too.

For example:
Assume that the time now is 2014-12-19 10:00("+8:00").
The "Start date" in "New issue" page and the "this day" in "Calendar" page are both 2014-12-18. And the expected date is 2014-12-19.

How to config the Redmine or server to make sure the date in "New issue" and "Calendar" shown in the time zone of "My Account" ?