I create unique numbers for forms using the following:
concat(substring(xdDate:Now(), 0, 5), substring(xdDate:Now(), 6, 2), substring(xdDate:Now(), 9, 2), substring(xdDate:Now(), 12, 2), substring(xdDate:Now(), 15, 2), substring(xdDate:Now(), 18, 2))
This returns date and time converted to a number i.e. 20140115191523
Play around with this, just display those parts that you require.
Hope this helps.
L Mac