From the three webmail clients provided by default on all SiteGround servers, SquirrelMail is the only one where you cannot adjust the Time zone settings.

In Horde the Time zone can be altered via Options -> Locale and Time. Choose the desired Time zone from the drop down menu then click the Save Options button.

In order to change the Time zone in RoundCube go to Personal Settings -> Preferences tab and select the desired Time zone from the drop down menu. Then click the Save button.