Windows regional time setting

I'm trying to customize an Access application that stores appointment date/times in a SharePoint list.

Unfortunately, I've got users all over the US that are using this application.  What I want to happen is that when the user selects 10am, and then saves the record, the value that actually gets saved will be in GMT.  So I need a way to convert a local time to GMT.  I'm assuming that the best way to do this is via an api call or by capturing a registry setting.  Just not sure how to accomplish this.

I'll either use the forms Current event or a conversion function in a query to convert the saved date/time back to local time for display purposes