Can you respond to any part or step of Zhonya's Hourglass?

Ruling: No, you cannot respond to Zhonya's Hourglass. It does not go on the chain because it replaces the act of killing a unit, and even when played from hidden at reaction speed, it is a permanent that resolves without priority.

Nuances:

• Permanents in general cannot be reacted to - they pass through the chain but don't allow responses
• This also applies to any ability with 'add' (like Malzahar, runes, seals)