This page discusses the problems of localisation.
Time
The time in OpenVPMS is set from the time in the 'server' machine - ie the one running the MySQL database and Tomcat application. If the user and the server are all in the same timezone, then there is no problem.
However, if you are using a hosting service somewhere else, then it may be necessary to set the server time to match the local time where your users are. [For example the OpenVPMS demo system runs on a server with its time set to GMT. If you log onto this from say Sydney, then early in the day you will find that 'today' on the server (and thus the default date on invoices you create) will be yesterday your time because GMT is 10 hours behind AEST.]
If you have users spread across multiple timezones, then you will need to educate those in the 'non-home' time zones that theirs will be different. Thus if you have a practice spread across the NSW/Queensland border but with headquarters in NSW, in summer your Surfers Paradise branch will be running an hour behind. In this case you probably want to run the appointment schedule for the Surfers branch on local time, but the timestamps on any activity will be NSW, not Queensland time.
Date Format
The date format used is set in the <TOMCAT_HOME>/webapps/openvpms/WEB-INF/classes/org/openvpms/web/resource/localisation/messages.properties file (or the local version - see Language below).
Specifically, if you need to use say a US date format, then you need to adjust the settings in this file. Note that as well as the date formats, you can also adjust the numeric formats if needed.
Language
OpenVPMS uses standard java i18n support so if you name your message properties file according to your locale settings, eg messages_fr.properties for France, then it will be used instead of the standard messages.properties.
Note that you can do minor language tweaks by editing the message.properties file. For example if you think that Organisation should really be Organization then you can change it here. [Though it would be better to create a message.properties-us file which includes both the spelling adjustments and the mm/dd/yy date formats.]