If you want this to be included / debated for the core. Which I seem as quite reasonable you might like to post a patch to open.silverstripe.com

I'm based in NYC and have needed US date formats for my customers. It's not comprehensive, but for adjusting the CMS I've used the following successfully:

/jsparty/calendar/calendar-setup.js

Change line 5 in initialise()
ifFormat : "%m/%d/%Y", // the date format

Change line 32 in setToToday()
this.value = ( date.getMonth() + 1   ) + '/' + date.getDate() + '/' + date.getYear();

/sapphire/forms/DateField.php

Change line 18 in setValue()
$this->value = preg_replace('/^([\d]{2,4})-([\d]{1,2})-([\d]{1,2})/','\\2/\\3/\\1', $val);

Change line 29 in dataValue()
return "$parts[3]-$parts[1]-$parts[2]";

It would be nice to see the d/m/y (m/d/y) type code moved to the _config.php file so it could be easily updated. Has anyone done that successfully?

Hope that helps someone!

-John