To change the timezone in arpReach, go to Setup / System Settings / System / General / Timezone.
You have to enter a valid PHP timezone code as defined in the PHP documentation here:
http://www.php.net/manual/en/timezones.america.php
Examples:
Europe/London
America/New_York (note the underscore)
The date and time the server is using based on this timezone code is shown at the bottom right of every screen in this software.