What is the test for a set-off against a counterclaim?

It was suggested on the argument by the defendant's counsel that the counterclaim related back to the date of the writ, namely, April 9, and that at that time the full six years from the date of the purchase of the seed, namely, May 1, 1921, had not run as against the counterclaim. It appears that in the case of a set-off the plaintiff must prove that the set-off was barred when the plaintiff commenced the action; the counterclaim, however, is different, and it is enough for the plaintiff to prove that the counterclaim was barred when it was pleaded. 19 Halsbury, 184; McGowan v. Middleton (1883) 11 Q.B.D. 464, 52 L.J.Q.B. 355.