diff --git a/datetime.h b/datetime.h
index 371cadbd..c057c76b 100644
--- a/datetime.h
+++ b/datetime.h
@@ -19,9 +19,6 @@ class date_t
 {
   date_t(const datetime_t& _when);
 
- protected:
-  std::time_t when;
-
  public:
   static date_t       now;
   static const char * formats[];
@@ -29,6 +26,8 @@ class date_t
   static std::string  input_format;
   static std::string  output_format;
 
+  std::time_t when;
+
   date_t() : when(0) {}
   date_t(const date_t& _when) : when(_when.when) {}
 
diff --git a/value.cc b/value.cc
index 641beb49..1ced0b96 100644
--- a/value.cc
+++ b/value.cc
@@ -1067,7 +1067,9 @@ void value_t::negate()
     *((long *) data) = - *((long *) data);
     break;
   case DATETIME:
-    throw new value_error("Cannot negate a date/time");
+    cast(INTEGER);
+    negate();
+    break;
   case AMOUNT:
     ((amount_t *) data)->negate();
     break;