At any rate we must assume that it was in the hands of Irenaeus about 185 A.D., and it will be not before this that the third book of the work 'Against Heresies' was written.

It will perhaps sufficiently satisfy all parties if we suppose that Irenaeus was engaged in writing his first three books between the years 182-188 A.D.But the name of Ptolemaeus is mentioned very near the beginning of the Preface; so that Irenaeus would be committing to paper the statement of his acquaintance with Ptolemaeus as early as 182 A.D.
This is however the last link in the chain.

Let us trace it a little further backwards.

Irenaeus' acquaintance with Ptolemaeus can hardly have been a fact of yesterday at the time when he wrote.

Ptolemaeus represented the 'Italian' branch of the Valentinian school, and therefore it seems a fair supposition that Irenaeus would come in contact with him during his visit to Rome in 178 A.D.; and the four years from that date to 182 A.D.