diff --git a/libs/ardour/ardour/export_smf_writer.h b/libs/ardour/ardour/export_smf_writer.h
index ea58b4b099..e3d5fdd3bd 100644
--- a/libs/ardour/ardour/export_smf_writer.h
+++ b/libs/ardour/ardour/export_smf_writer.h
@@ -38,6 +38,7 @@ public:
 	~ExportSMFWriter ();
 
 	int init (std::string const& path, samplepos_t);
+	Temporal::Beats duration() const;
 
 	void process (MidiBuffer const&, sampleoffset_t, samplecnt_t, bool);
 
diff --git a/libs/ardour/ardour/smf_source.h b/libs/ardour/ardour/smf_source.h
index 19cfcc1bef..b40aab9dd8 100644
--- a/libs/ardour/ardour/smf_source.h
+++ b/libs/ardour/ardour/smf_source.h
@@ -81,6 +81,8 @@ public:
 
 	void render (const ReaderLock& lock, Evoral::EventSink<Temporal::Beats>& dst);
 
+	Temporal::Beats duration() const;
+
   protected:
 	void close ();
 	void flush_midi (const WriterLock& lock);
diff --git a/libs/ardour/export_smf_writer.cc b/libs/ardour/export_smf_writer.cc
index 16aeff975b..cfdbe2d1d4 100644
--- a/libs/ardour/export_smf_writer.cc
+++ b/libs/ardour/export_smf_writer.cc
@@ -95,3 +95,9 @@ ExportSMFWriter::process (MidiBuffer const& buf, sampleoffset_t off, samplecnt_t
 		_pos += n_samples;
 	}
 }
+
+Temporal::Beats
+ExportSMFWriter::duration() const
+{
+	return std::numeric_limits<Temporal::Beats>::max ();
+}
diff --git a/libs/ardour/smf_source.cc b/libs/ardour/smf_source.cc
index a08e7ac4e0..488352e193 100644
--- a/libs/ardour/smf_source.cc
+++ b/libs/ardour/smf_source.cc
@@ -462,10 +462,15 @@ void
 SMFSource::update_length (timepos_t const & dur)
 {
 	assert (!_length || (_length.time_domain() == dur.time_domain()));
-	Evoral::SMF::set_duration (dur.beats());
 	_length = dur;
 }
 
+Temporal::Beats
+SMFSource::duration() const
+{
+	return _length.beats ();
+}
+
 /** Append an event with a timestamp in beats */
 void
 SMFSource::append_event_beats (const WriterLock&   lock,
@@ -643,7 +648,7 @@ SMFSource::mark_midi_streaming_write_completed (const WriterLock& lm, Evoral::Se
 	}
 
 	try {
-		Evoral::SMF::set_duration (duration.beats());
+		update_length (duration.distance());
 		Evoral::SMF::end_write (_path);
 	} catch (std::exception & e) {
 		error << string_compose (_("Exception while writing %1, file may be corrupt/unusable"), _path) << endmsg;
diff --git a/libs/evoral/SMF.cc b/libs/evoral/SMF.cc
index caadc2d918..42718777cf 100644
--- a/libs/evoral/SMF.cc
+++ b/libs/evoral/SMF.cc
@@ -516,15 +516,24 @@ SMF::end_write (string const & path)
 		return;
 	}
 
-
-
-
 	FILE* f = g_fopen (path.c_str(), "w+b");
 	if (f == 0) {
 		throw FileError (path);
 	}
 
-	if (smf_save(_smf, f) != 0) {
+	Temporal::Beats b = duration();
+
+	if (b != std::numeric_limits<Temporal::Beats>::max()) {
+
+		int64_t their_pulses = b.to_ticks (_smf->ppqn);
+
+		for (uint16_t n = 0; n < _smf->number_of_tracks; ++n) {
+			smf_track_t* trk = smf_get_track_by_number (_smf, n+1);
+			(void) smf_track_add_eot_pulses (trk, their_pulses);
+		}
+	}
+
+	if (smf_save (_smf, f) != 0) {
 		fclose(f);
 		throw FileError (path);
 	}
@@ -532,23 +541,8 @@ SMF::end_write (string const & path)
 	fclose(f);
 }
 
-void
-SMF::set_duration (Temporal::Beats const & b)
-{
-	if (!_smf) {
-		return;
-	}
-
-	size_t their_pulses = b.to_ticks (ppqn());
-
-	for (uint16_t n = 0; n < _smf->number_of_tracks; ++n) {
-		smf_track_t* trk = smf_get_track_by_number (_smf, n+1);
-		(void) smf_track_add_eot_pulses (trk, their_pulses);
-	}
-}
-
 Temporal::Beats
-SMF::duration () const
+SMF::file_duration () const
 {
 	if (!_smf) {
 		return Temporal::Beats();
diff --git a/libs/evoral/evoral/SMF.h b/libs/evoral/evoral/SMF.h
index 57e58db09a..fd664df72a 100644
--- a/libs/evoral/evoral/SMF.h
+++ b/libs/evoral/evoral/SMF.h
@@ -27,6 +27,8 @@
 #include <memory>
 #include <set>
 
+#include "temporal/beats.h"
+
 #include "evoral/visibility.h"
 #include "evoral/types.h"
 
@@ -39,7 +41,6 @@ typedef smf_tempo_struct smf_tempo_t;
 
 namespace Temporal {
 	class TempoMap;
-	class Beats;
 }
 
 namespace Evoral {
@@ -70,6 +71,8 @@ public:
 	SMF();
 	virtual ~SMF();
 
+	virtual Temporal::Beats duration() const { return std::numeric_limits<Temporal::Beats>::max(); }
+
 	static bool test(const std::string& path);
 	int open (const std::string& path, int track = 1, bool scan = true);
 	// XXX 19200 = 10 * Temporal::ticks_per_beat
@@ -96,8 +99,7 @@ public:
 
 	int smf_format () const;
 
-	void set_duration (Temporal::Beats const &);
-	Temporal::Beats duration() const;
+	Temporal::Beats file_duration() const;
 
 	int num_channels () const { return _num_channels; }
 	typedef std::bitset<16> UsedChannels;