The date code is often (but not always) a number stamped on the seat tube at/near the top
Date stamped on the seat tube of my Alan is ALAN1080 which is October 1980
3J 52X54 stamped on the BB shell which is the frame size (unsure what the 3J is)
564XXX stamped on the BB shell which is the serial number
It might be worth emailing ALAN themselves (alfa@alanbike.it). From what I've read on some internet forums they seem to keep a good database of frame serial numbers and are usually quite helpful if you email them your frame number.