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!
