On the third time the spectre produced the knife, but at _this_ information the duke only laughed.

Six weeks later he was stabbed.

Douch makes the whole affair pass immediately before the assassination.

"And Mr.Parker died soon after," as the ghost had foretold to him.
Finally, Clarendon makes the appearances set in six months before Felton slew the duke.

The percipient, unnamed, was in bed.