Implement the first part of the calendar support for C++20. This is still incomplete; there will be more patches coming. Reviewed as D51762

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@344529 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/utilities/time/days.pass.cpp

@@ -0,0 +1,28 @@
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+
+// <chrono>
+
+// using days = duration<signed integer type of at least 25 bits, ratio_multiply<ratio<24>, hours::period>>;
+
+#include <chrono>
+#include <type_traits>
+#include <limits>
+
+int main()
+{
+    typedef std::chrono::days D;
+    typedef D::rep Rep;
+    typedef D::period Period;
+    static_assert(std::is_signed<Rep>::value, "");
+    static_assert(std::is_integral<Rep>::value, "");
+    static_assert(std::numeric_limits<Rep>::digits >= 25, "");
+    static_assert(std::is_same_v<Period, std::ratio_multiply<std::ratio<24>, std::chrono::hours::period>>, "");
+}