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This looks to be an issue with dates before the Gregorian calendar start (15-october-1582), Need to determine why this works when swift-corelibs-foundation is running on macOS but not Linux, which shares the same CoreFoundation Calendar code but a different ICU

Looking at it further, the difference starts with the return value of formatter.date(from🙂 from the ISO8601DateFormatter

The following run on macOS v Linux shows the difference (using swift-DEVELOPMENT-SNAPSHOT-2020-01-24-a-ubuntu18.04 on Linux)

import Foundation

func test(_ testDateStr: String) {
  let formatter = ISO8601DateFormatter()
  let date = formatter.date(from: testDateStr)!
  print(testDateStr, "->", date.timeIntervalSinceReferenceDate)
}

test("1230-10-08T12:21:18Z")
test("1500-10-08T12:21:18Z")
test("1580-10-08T12:21:18Z")
test("1582-09-08T12:21:18Z")
test("1582-10-02T12:21:18Z")
test("1582-10-16T12:21:18Z")
test("1582-11-08T12:21:18Z")
test("1583-10-08T12:21:18Z")
test("1584-10-08T12:21:18Z")
test("1600-10-08T12:21:18Z")

All of the dates before the start of the Gregorian calendar (1582-10-15) show a difference, so I think its just the ISO8601Date formatter than needs to be fixed first.

the following is true in the swift console on macOS:

// Welcome to Apple Swift version 5.6.1 (swiftlang-5.6.0.323.66 clang-1316.0.20.12).
import Foundation
ISO8601DateFormatter.init().date(
  from: "1582-10-14T12:00:00Z"
) ==  ISO8601DateFormatter.init().date(
  from: "1582-10-24T12:00:00Z"
)

obviously 1582-10-14T12:00:00Z and 1582-10-24T12:00:00Z are not the same date. there is a bug in the date formatter in macOS where it seems to be using the Julian calendar for dates prior to the introduction of the Gregorian. this is not proper according to ISO 8601.