By default, webmail applications use the host server’s time zone. If you are in a different time zone, you can change this by login to your webmail area. Here I give you a guideline to change the time zone on Horde, Roundcube and SquirrelMail interfaces.
To change the time zone in Horde, follow below steps:
From your webmail logged interface, click on ‘Horde‘.
Click on the gear icon on the top middle portion of the page (shows as in below image)
Then go to ‘Preferences‘ > ‘Global Preferences‘ >
Under Your Information, click ‘Locale and Time‘.
Select your time zone, and then click Save Options. In that page itself you can set the preferred language too.
To change the time zone in Roundcube, follow below steps:
From your webmail logged interface, click on ‘Roundcube‘.
Click on ‘Settings’ (The gear icon on top right hand side, see the below image). 
The click on ‘User Interface‘.
Select your time zone, format, date format. Click on the Save button to take effect the changes.
To change the time zone in SquirrelMail, follow below steps:
Log in to SquirrelMail.
Click on ‘Options‘ link available on top middle portion.
Then click on ‘Personal Information‘.
Under Timezone Options, select your time zone, and then click Submit.
