The date of manufacture is a 4 digit number on the side-wall comprising week and year - eg '3605' would be week '36' of year '05'
Here's the tyre in question...
For some reason you can't really see the bulging/distortion, but you can see the cracks in the rubber at the centre of the bulge area (right side of treads)
The car has just passed its annual inspection (MOT) and did have an 'advisory' which mentioned 'slight cracking' in this tyre. I thought they meant the slight crazing in the sidewalls - had I had any idea it was as bad as it is, I would have done something about it immediately!
Maybe it was the higher pressure that high-lighted the problem?
No harm done, luckily, but running old tyres could have proved to be a very false economy!
BTW running at 30psi instead of 40psi (4 psi below max) appeared to make a difference of 2-3mpg at steady speeds according to a quick and dirty test using my Ultraguage!