diff --git a/src/ISO8601.cpp b/src/ISO8601.cpp
index 6fa9df6c1..0295a96b6 100644
--- a/src/ISO8601.cpp
+++ b/src/ISO8601.cpp
@@ -1201,6 +1201,20 @@ time_t ISO8601d::operator- (const ISO8601d& rhs)
   return _date - rhs._date;
 }
 
+////////////////////////////////////////////////////////////////////////////////
+// Prefix decrement by one day.
+void ISO8601d::operator-- ()
+{
+  ISO8601d yesterday = startOfDay () - 1;
+  yesterday = ISO8601d (yesterday.month (),
+                        yesterday.day (),
+                        yesterday.year (),
+                        hour (),
+                        minute (),
+                        second ());
+  _date = yesterday._date;
+}
+
 ////////////////////////////////////////////////////////////////////////////////
 void ISO8601p::clear ()
 {
diff --git a/src/ISO8601.h b/src/ISO8601.h
index d4cd49768..85833640d 100644
--- a/src/ISO8601.h
+++ b/src/ISO8601.h
@@ -85,6 +85,7 @@ public:
   ISO8601d& operator+= (const int);
   ISO8601d& operator-= (const int);
   time_t operator- (const ISO8601d&);
+  void operator-- ();    // Prefix
 
 private:
   void clear ();