Conversation so far:
What was the Mirror challenge?
challenged Houdini to escape from special handcuffs that it claimed had taken Nathaniel Hart.
Who challenged Houdini?
London Daily Mirror newspaper.
Who was Nathaniel Hart?
a locksmith from Birmingham.
What was special about the handcuffs?
five years to make.

Question: When did he complete the challenge?
Self-contained rewrite: When did Harry Houdini complete the mirror challenge?