This is almost certainly a time zone issue in OWA.
Outlook adopts the time zone of the host operating system; both of your client machines are configured for the same time zone, so the appointment is displayed at the same time on both machines in Outlook.
Outlook Web Access can't make the assumption that the client machine where the web browser is running is in the same time zone as the server, so the Outlook Web Access client provides a user-configurable option where the user can set their current time zone. This information is stored for each user.
Sign in to Outlook Web Access, navigate to the Options page and select the Date & Time Formats section to check the current time zone setting. Select the correct time zone and click the Save button and you should be able to avoid showing up to meetings four hours early!
Note that these options in Outlook Web Access are stored per-user, so the user experiencing the problem will need to check and modify this setting.
We currently use Exchange 2007; Outlook Web Access for Exchange 2007 calls this Options section "Regional Settings" instead of "Date & Time Formats" but it works the same.
I believe that users are prompted to configure their time zone the very first time they sign in to Outlook Web Access; however, most users of Outlook Web Access aren't aware of this setting, so if is set incorrectly or needs to be changed, most people don't even know to look for it...
The problem was caused because our firewall was somehow configured to act as DHCP server, in addition to the regular DHCP server we had. As a result, the computers that received their IP address from the regular DHCP server worked fine. Those that received their IP address from the firewall didn't work!
Best Answer
These are both possible. For Outlook 2007, the process is called Outlook Anywhere and for Windows Mobile, it is called Activesync & Direct Push. Both of these are available "out of the box" with Exchange 2007 but do require a little configuration on the server side (Exchange 2003 SP2 also provides these capabilities).
For Outlook, if you do not have the autodiscover service working, then you need to manually configure Outlook. To do this go to Tools > Account settings and double-click on your account name. Click the More Settings button, go to the Connection and check the box under Outlook Anywhere.
Then select the Exchange Proxy Settings box and in the URL type in the address for your OWA server. Check the two SSL boxes assuming SSL is configured correctly on OWA. Then turn on the setting for HTTP on slow networks. OK your way out of the menus and you should be set (Outlook may require a restart).
For Activesync, simply enter in the address of your OWA server, enter your authentication information, and enable SSL.
If you need help with the server-side, then we will need to know what version of Exchange you are running.