System.Globalization

 namespace System.Globalization {
     public class CompareInfo : IDeserializationCallback {
+        public int GetHashCode(ReadOnlySpan<char> source, CompareOptions options);
     }
+    public static class ISOWeek {
+        public static int GetWeekOfYear(DateTime date);
+        public static int GetWeeksInYear(int year);
+        public static int GetYear(DateTime date);
+        public static DateTime GetYearEnd(int year);
+        public static DateTime GetYearStart(int year);
+        public static DateTime ToDateTime(int year, int week, DayOfWeek dayOfWeek);
+    }
 }