You can easily change the timezone in Horde.
1.Simply login to the webmail client Horde. Find the Main Navigation Menu at the end of which is placed the Settings Icon. Choose Preferences from the menu and then go to Global Preferences.
2.Find the Locale and Time section from where you will be able to set your preferred language, timezone and date preferences.
3.And once you select the ones desired you can just save the changes.