This is my small invoice "template" that I usually use to write my invoices because I've often write just once a month a invoice. The project inside of the IT environment often take more than six month therefore it is good enough for me. :-)
BTW, for Germans the sentence "...sonst tritt Verzug ein." is importend if some customer does not pay your bill. In this case you can get interests from the date of the bill and not from the first reminder. ;-)