Login to your cpanel webmail account and select your preferred client and follow the steps for your client
Horde
Once your in the Horde client do the following
1. Hover over the gear icon on the top bar and select Global Preferences
2. Click on Locale and Time
3. Select the time zone that you would like to use then click Save Options.
Roundcube
Once your in the Roundcube client do the following
1. Click Settings
2. Click User Interface
3. Select the time zone that you would like to use
4. Click Save
SquirrelMail
Once your in the SquirrelMail client do the following
1. Click Options
2. Click Personal Information
3. Under Timezone Options, Select the time zone that you would like to use
4. Click Submit