Swift DateFormatter issue

Question

I use below code to format a Date and get a wrong date string.

    let formatter = DateFormatter()
    formatter.timeZone = TimeZone(identifier: "UTC")
    formatter.dateFormat = "YYYY-MM-dd"
    
    let dateStr = formatter.string(from: date) // 2012-12-31 07:52:49 +0000

then dateStr is '2013-12-31'.

Anyone has ideas about it?