Why am I getting "Expressions are not allowed at the top level" error?

If I understand correctly, this code is inside a class definition or an extension of Date ? Please show complete code, that will make it much easier to help you.

Then, code as

// Today in milliseconds
Date().millisecondsSince1970 // 1641554695757

// The Date for 1 000 000 000 000 milliseconds
print(Date(milliseconds: 1_000_000_000_000)) // 2001-09-09 01:46:40 +0000

must be inside a func, not directly at class level as a declaration.

So you should write:

// Whatever header is…
  var millisecondsSince1970: Int64 {
    Int64((self.timeIntervalSince1970 * 1000.0).rounded())
  }
   
  init(milliseconds: Int64) {
    self = Date(timeIntervalSince1970: TimeInterval(milliseconds) / 1000)
  }

func displayDates() {
  // Today in milliseconds
  let dateAsms = Date().millisecondsSince1970 // 1641554695757  Do whatever you need with this 

   // The Date for 1 000 000 000 000 milliseconds
    print(Date(milliseconds: 1_000_000_000_000)) // 2001-09-09 01:46:40 +0000
}