This is disclaimer about how JoomlaStats deal with time and timezones.
For performance JoomlaStats since v3.0 store all dates and times in database in Joomla CMS local timezone for anonymous users.
Anonymous users are users that browse front page without login - most users and all bots (eg. google bot)
Set time zone for anonymous user in Joomla CMS v1.5:
Joomla CMS backend -> tool bar menu "Site" position -> "Global Configuration" -> "Server" tab -> "Locale Settings" box -> "Time Zone" property