This is good question, my son got citizenship through me, I applied his passport and it was approved. I also applied N600 still pending. So out of curiosity, I call the passport office to confirm the date my son became citizen, the lady told me is the date the passport was issued, the following day, I call USCIS and the guy told me my son became citizen the same day I was citizen. A week later, I call USCIS again and asked to speak with Tier 2, he told me legally, it's the date the passport was issue so am just 😕 confuse. I intend to go by the date on his passport