The timestamp on content is determined by a user's time zone settings. Admins set a default time zone in the Administration, which is applied to all new users. This time zone remains effective until the user decides to change their account settings.
You can find the settings for the default time zone in Administration > System settings > Registration/Account. Any changes to the default time zone will only affect new users created after the change.
If a user changes their time zone in their account settings, it overrides the default time zone.