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:

In SquirrelMail:
Click on “Options” ->  “Personal Information”
In Horde:
Click on “Options” -> “Global Options” -> on”Locale and Time”

In Roundcube:

Click on “Settings” -> “User Interface”

3. Select the timezone you need.