Implement Date.description in pure Swift

# Motivation

The current implementation of `Date.description` is depending on the what platform we are running on. On Darwin it used the CFFoundation backed implementation and on Linux/Windows it used libc APIs to do the date calculations. This caused build issues on Windows and the libc APIs are not 32bit clean.

# Changes

This PR changes the implementation by using the methods implemented in swift-certificates that provide timestamp to UTC time calculations. These were added in swift-certificates for the exact same reason. Furthermore, it removes the conditional usage of CFFoundation since the output is the same.

# Result

Swift only implementation that works cross platform for `Date.description` which also should perform better than any libc API calls.

Align the tests

Code review

Code review

Fix compiler error

NOTICE.txt

@@ -0,0 +1,43 @@
+
+The SwiftFoundation Project
+=====================
+
+Please visit the SwiftFoundation web site for more information:
+
+* https://github.com/apple/swift-foundation
+
+Copyright 2023 The SwiftFoundation Project
+
+The SwiftFoundation Project licenses this file to you under the Apache License,
+version 2.0 (the "License"); you may not use this file except in compliance
+with the License. You may obtain a copy of the License at:
+
+https://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+License for the specific language governing permissions and limitations
+under the License.
+
+Also, please refer to each LICENSE.txt file, which is located in
+the 'license' directory of the distribution file, for the license terms of the
+components that this product depends on.
+
+---
+
+This product contains derivations of the time calculations from SwiftCertificates.
+
+* LICENSE (Apache License 2.0):
+* https://www.apache.org/licenses/LICENSE-2.0
+* HOMEPAGE:
+* https://github.com/apple/swift-certificates
+
+---
+
+This product contains code to calculate and decompose UNIX timestamps derived from musl libc.
+
+* LICENSE (MIT):
+* https://git.musl-libc.org/cgit/musl/tree/COPYRIGHT
+* HOMEPAGE:
+* https://musl.libc.org

Sources/FoundationEssentials/Date.swift

@@ -233,24 +233,14 @@ extension Date {
 
 @available(macOS 10.10, iOS 8.0, watchOS 2.0, tvOS 9.0, *)
 extension Date : CustomDebugStringConvertible, CustomStringConvertible, CustomReflectable {
-// For backwards compatibility, the Darwin version of this method is left alone
-// because it uses `NSDateFormatter` and may behave slightly differently.
-#if !FOUNDATION_FRAMEWORK
     /// A string representation of the date object (read-only).
-    /// The representation is useful for debugging only.
+    /// The representation is useful for debugging only and might change over time. It currently returns
+    /// a representation in UTC time with a Gregorian calendar.
     /// There are a number of options to acquire a formatted string for a date including: date formatters
     /// (see [NSDateFormatter](//apple_ref/occ/cl/NSDateFormatter) and
     /// [Data Formatting Guide](//apple_ref/doc/uid/10000029i)), and the `Date`
     /// function `description(locale:)`.
     public var description: String {
-        // NSDate uses the constant format `uuuu-MM-dd HH:mm:ss '+0000'`
-
-        // Glibc needs a non-standard format option to pad %Y to 4 digits
-#if canImport(Glibc)
-        let format = "%4Y-%m-%d %H:%M:%S +0000"
-#else
-        let format = "%Y-%m-%d %H:%M:%S +0000"
-#endif
         let unavailable = "<description unavailable>"
 
         guard self >= Date.distantPast else {
@@ -260,36 +250,12 @@ extension Date : CustomDebugStringConvertible, CustomStringConvertible, CustomRe
             return unavailable
         }
 
-        var info = tm()
-#if os(Windows)
-        var time = __time64_t(self.timeIntervalSince1970)
-        let errno: errno_t = _gmtime64_s(&info, &time)
-        guard errno == 0 else { return unavailable }
-#else
-        var time = time_t(self.timeIntervalSince1970)
-        gmtime_r(&time, &info)
-#endif
-
-        // This allocates stack space for range of 10^102 years
-        // That's more than Date currently supports.
-        let bufferSize = 128
-        return withUnsafeTemporaryAllocation(of: CChar.self, capacity: bufferSize) { buffer in
-            guard let ptr = buffer.baseAddress else {
-                return unavailable
-            }
+        // We are casting to Int64 here which means we are rounding down.
+        // This is fine since we only need second granularity here for the description
+        let (year, month, day, hours, minutes, seconds) = Int64(self.timeIntervalSince1970).utcDateFromTimestamp
 
-            guard strftime(ptr, bufferSize, format, &info) != 0 else {
-                return unavailable
-            }
-
-            guard let result = String(validatingUTF8: ptr) else {
-                return unavailable
-            }
-
-            return result
-        }
+        return "\(_zeroPad: year, _toWidth: 4)-\(_zeroPad: month, _toWidth: 2)-\(_zeroPad: day, _toWidth: 2) \(_zeroPad: hours, _toWidth: 2):\(_zeroPad: minutes, _toWidth: 2):\(_zeroPad: seconds, _toWidth: 2) +0000"
     }
-#endif // !FOUNDATION_FRAMEWORK
 
     public var debugDescription: String {
         return description
@@ -388,3 +354,15 @@ extension Date {
         Date.validCalendarRange.contains(self)
     }
 }
+// MARK: - String interpolation helper for description property
+extension DefaultStringInterpolation {
+    fileprivate mutating func appendInterpolation(_zeroPad value: Int, _toWidth width: Int) {
+      precondition(width > 0)
+      let representation = String(value)
+      let padding = width &- representation.utf8.count
+      if padding > 0 {
+          appendLiteral(String(repeating: "0", count: padding))
+      }
+      appendLiteral(representation)
+    }
+}