Filename Date Format SharePoint 2013

I am trying to change the filename date format for an InfoPath form generated item.

Currently I am using this file name formula: concat("Backup", "-", today())

Which results in this filename Backup-2015-02-12

However having changed my locale settings to United Kingdom the date format has changed for another field on my form but not on this one.

I need the file name to display as Backup-12-02-2015

Thanks.