Custom order ID

I need to Generate a custom order ID number which is a combination of other variables specifically date. My order number is made up of:

=> mmddyy-0001

Where mm = month i.e. 05 for may
dd = day i.e. 25 for 25th
yy = year i.e. 14 for 2014.

How can i generate a unique order with such a format.

Thanks