How to change Timezones on common webmail clients

Login to webmail of the domain

For Squirrelmail webmail
click on Options >> – Personal Information >> Timezone Options. update your time & Save it !

For roundcube webmail
click on setting >> Preferences >> User Interface >> Timezone Options. update your time & Save it

For Horde webmail
Click on option >> Locale and Time Set your preferred language, timezone and date options.& save it >>

For doing this an account wide or domain wise, its not possible i think so,

Hope this helps!