For Horde
Step 1: Login to Horde
Step 2: In the left-hand click Options, and then click Global Options.
Step 3: Click Locale and Time under your information.
Step 4: Select the time zone you wish to change, and then click Save Options.
For Roundcube
Step 1: Log in to Roundcube.
Step 2: Click Settings option, in that Click User Interface.
Step3: Select the time zone you wish to change, and then click Save Options.
For SquirrelMail
Step 1: Log in to SquirrelMail.
Step 2: Click Options left pane and then Click Personal Information.
Step 3: Under Time zone Options, Select the time zone you wish to change, and then click Submit.