If you want to change the timezone for your webmail as per your region onn your server, please follow the below steps:
1. Log into SquirrelMail/Horde/Roundcube webmail client with the user that you would like to change the time zone for.
2. Access the following option:
Click on “Options” -> “Personal Information”
Click on “Options” -> “Global Options” -> on”Locale and Time”
Click on “Settings” -> “User Interface”
3. Select the timezone you need.