I suggest you study the Glencoyne site, which is my go to site for identifying old Land Rover engines. Link below,
http://www.glencoyne.co.uk/engno.htm
I don't know any way of "dating" the engines, assuming that by that you mean establishing the exact date at which the engine was built.
But I do know that 2.5 Turbodiesels (19j) weren't fitted for all that long, maybe from 1986 up to early 1990, after which Defenders fitted with 200Tdi appeared.
Also, according to Glencoyne, the engines were fitted with a different block, and, from my own experience, with pistons with coated crowns, from Engine No. 19j27515c. This to overcome some of the many problems that early engines experienced.
So it would appear that your engine is a later one, or at least after the mods were brought in.
Although neither I, nor, it seems, Glencoyne, know what date that engine was fitted.
Might I ask why you are interested in this? Parts can be ordered from Turners, or other reputable suppliers, by quoting the Engine Number.
And I assume you know when the vehicle was registered, which would indicate that the engine was built in the few months preceding that date.