diff --git a/src/ISO8601.cpp b/src/ISO8601.cpp
index 8bbfabc36..ae78d5a61 100644
--- a/src/ISO8601.cpp
+++ b/src/ISO8601.cpp
@@ -1004,13 +1004,13 @@ void ISO8601d::resolve ()
 }
 
 ////////////////////////////////////////////////////////////////////////////////
-time_t ISO8601d::toEpoch ()
+time_t ISO8601d::toEpoch () const
 {
   return _date;
 }
 
 ////////////////////////////////////////////////////////////////////////////////
-std::string ISO8601d::toEpochString ()
+std::string ISO8601d::toEpochString () const
 {
   std::stringstream epoch;
   epoch << _date;
@@ -1019,7 +1019,7 @@ std::string ISO8601d::toEpochString ()
 
 ////////////////////////////////////////////////////////////////////////////////
 // 19980119T070000Z =  YYYYMMDDThhmmssZ
-std::string ISO8601d::toISO ()
+std::string ISO8601d::toISO () const
 {
   struct tm* t = gmtime (&_date);
 
@@ -1037,13 +1037,13 @@ std::string ISO8601d::toISO ()
 }
 
 ////////////////////////////////////////////////////////////////////////////////
-double ISO8601d::toJulian ()
+double ISO8601d::toJulian () const
 {
   return (_date / 86400.0) + 2440587.5;
 }
 
 ////////////////////////////////////////////////////////////////////////////////
-void ISO8601d::toMDY (int& m, int& d, int& y)
+void ISO8601d::toMDY (int& m, int& d, int& y) const
 {
   struct tm* t = localtime (&_date);
 
diff --git a/src/ISO8601.h b/src/ISO8601.h
index 6af7f96f6..48aeeb528 100644
--- a/src/ISO8601.h
+++ b/src/ISO8601.h
@@ -47,11 +47,11 @@ public:
   operator time_t () const;
   bool parse (const std::string&, std::string::size_type&, const std::string& format = "");
 
-  time_t toEpoch ();
-  std::string toEpochString ();
-  std::string toISO ();
-  double toJulian ();
-  void toMDY (int&, int&, int&);
+  time_t toEpoch () const;
+  std::string toEpochString () const;
+  std::string toISO () const;
+  double toJulian () const;
+  void toMDY (int&, int&, int&) const;
   const std::string toString (const std::string& format = "m/d/Y") const;
 
   ISO8601d startOfDay () const;