Initial unit test submission.

test/date_test/daysmday.fail.cpp

@@ -0,0 +1,14 @@
+// Howard Hinnant
+// This work is licensed under a Creative Commons Attribution 4.0 International License.
+// http://creativecommons.org/licenses/by/4.0/
+
+// days - day not allowed
+
+#include "date.h"
+
+int
+main()
+{
+    using namespace date;
+    auto x = days{3} - 7_d;
+}