ios – What is the explanation of observing a constant data change in SwiftUI View

When I study the SwiftUI Apprentice book of Kodeco, I come across the below code snippet.

struct CountdownView: View {
  let date: Date
  @Binding var timeRemaining: Int
  let size: Double

  var body: some View {
    Text("\(timeRemaining)")  // 5
      .font(.system(size: size, design: .rounded))
      .padding()
      .onChange(of: date) { _ in  // 6
        timeRemaining -= 1
      }
  }
}

struct TimerView: View {
  @State private var timeRemaining: Int = 3  // 1
  @Binding var timerDone: Bool  // 2
  let size: Double

  var body: some View {
    TimelineView(  // 3
      .animation(
        minimumInterval: 1.0,
        paused: timeRemaining <= 0)) { context in
          CountdownView(  // 4
            date: context.date,
            timeRemaining: $timeRemaining,
            size: size)
        }
        .onChange(of: timeRemaining) { _ in
          if timeRemaining < 1 {
            timerDone = true  // 7
          }
        }
  }
}

struct TimerView_Previews: PreviewProvider {
  static var previews: some View {
    TimerView(timerDone: .constant(false), size: 90)
  }
}

When I examine the code snippet, I firstly expected to be recreated CountdownView. But there would be no change on date property in that case. Then, I added onAppear modifier to CountdownView to detect the recreating of the view. As a result, it is created only once. I come up with a question to help me to understand the view rending mechanism of SwiftUI, on which I still work that although date property is a constant and does not use @Binding how is it possible to observe the change of a data that is declared as a constant in SwiftUI?