diff --git a/pcbnew/eagle_plugin.h b/pcbnew/eagle_plugin.h
index 8ee00d5768..d38ef63c1e 100644
--- a/pcbnew/eagle_plugin.h
+++ b/pcbnew/eagle_plugin.h
@@ -98,6 +98,11 @@ public:
     MODULE* FootprintLoad( const wxString& aLibraryPath, const wxString& aFootprintName,
             const PROPERTIES* aProperties = NULL ) override;
 
+    long long GetLibraryTimestamp() const override
+    {
+        return getModificationTime( m_lib_path ).GetValue().GetValue();
+    }
+
     bool IsFootprintLibWritable( const wxString& aLibraryPath ) override
     {
         return false;   // until someone writes others like FootprintSave(), etc.
diff --git a/pcbnew/gpcb_plugin.cpp b/pcbnew/gpcb_plugin.cpp
index ccc75d24b2..fe0a68cd6f 100644
--- a/pcbnew/gpcb_plugin.cpp
+++ b/pcbnew/gpcb_plugin.cpp
@@ -280,7 +280,7 @@ GPCB_FPL_CACHE::GPCB_FPL_CACHE( GPCB_PLUGIN* aOwner, const wxString& aLibraryPat
 wxDateTime GPCB_FPL_CACHE::GetLibModificationTime() const
 {
     if( !m_lib_path.DirExists() )
-        return wxDateTime::Now();
+        return wxDateTime( 0.0 );
 
     return m_lib_path.GetModificationTime();
 }
@@ -1131,6 +1131,16 @@ bool GPCB_PLUGIN::FootprintLibDelete( const wxString& aLibraryPath, const PROPER
 }
 
 
+long long GPCB_PLUGIN::GetLibraryTimestamp() const
+{
+    // If we have no cache, return a number which won't match any stored timestamps
+    if( !m_cache )
+        return wxDateTime::Now().GetValue().GetValue();
+
+    return m_cache->GetLibModificationTime().GetValue().GetValue();
+}
+
+
 bool GPCB_PLUGIN::IsFootprintLibWritable( const wxString& aLibraryPath )
 {
     LOCALE_IO   toggle;
diff --git a/pcbnew/gpcb_plugin.h b/pcbnew/gpcb_plugin.h
index 2d73231649..c5c898b1bf 100644
--- a/pcbnew/gpcb_plugin.h
+++ b/pcbnew/gpcb_plugin.h
@@ -74,6 +74,8 @@ public:
     bool FootprintLibDelete( const wxString& aLibraryPath,
                              const PROPERTIES* aProperties = NULL ) override;
 
+    long long GetLibraryTimestamp() const override;
+
     bool IsFootprintLibWritable( const wxString& aLibraryPath ) override;
 
     //-----</PLUGIN API>--------------------------------------------------------
diff --git a/pcbnew/io_mgr.h b/pcbnew/io_mgr.h
index 5e3efcebc2..3f5efb4b0e 100644
--- a/pcbnew/io_mgr.h
+++ b/pcbnew/io_mgr.h
@@ -358,11 +358,7 @@ public:
      * directory).
      * Timestamps should not be considered ordered; they either match or they don't.
      */
-    virtual long long GetLibraryTimestamp() const
-    {
-        // Default implementation.
-        return 0;
-    }
+    virtual long long GetLibraryTimestamp() const = 0;
 
     /**
      * Function PrefetchLib
diff --git a/pcbnew/kicad_plugin.cpp b/pcbnew/kicad_plugin.cpp
index 3275be545b..891a22f325 100644
--- a/pcbnew/kicad_plugin.cpp
+++ b/pcbnew/kicad_plugin.cpp
@@ -122,9 +122,10 @@ class FP_CACHE
     wxFileName      m_lib_path;     /// The path of the library.
     MODULE_MAP      m_modules;      /// Map of footprint file name per MODULE*.
 
-    bool            m_cache_dirty;      // Stored separately because it's expensive
-                                        // to check m_cache_timestamp against file.
-    long long       m_cache_timestamp;  // A timestamp for the footprint file.
+    bool            m_cache_dirty;      // Stored separately because it's expensive to check
+                                        // m_cache_timestamp against all the files.
+    long long       m_cache_timestamp;  // A hash of the timestamps for all the footprint
+                                        // files.
 
 public:
     FP_CACHE( PCB_IO* aOwner, const wxString& aLibraryPath );
@@ -2131,7 +2132,7 @@ void PCB_IO::FootprintDelete( const wxString& aLibraryPath, const wxString& aFoo
 
 long long PCB_IO::GetLibraryTimestamp() const
 {
-    // If we have not cache, return a number which won't match any stored timestamps
+    // If we have no cache, return a number which won't match any stored timestamps
     if( !m_cache )
         return wxDateTime::Now().GetValue().GetValue();
 
diff --git a/pcbnew/legacy_plugin.cpp b/pcbnew/legacy_plugin.cpp
index 39237adac8..7e20aa936d 100644
--- a/pcbnew/legacy_plugin.cpp
+++ b/pcbnew/legacy_plugin.cpp
@@ -3383,6 +3383,16 @@ void LP_CACHE::LoadModules( LINE_READER* aReader )
 }
 
 
+long long LEGACY_PLUGIN::GetLibraryTimestamp() const
+{
+    // If we have no cache, return a number which won't match any stored timestamps
+    if( !m_cache )
+        return wxDateTime::Now().GetValue().GetValue();
+
+    return m_cache->GetLibModificationTime().GetValue().GetValue();
+}
+
+
 void LEGACY_PLUGIN::cacheLib( const wxString& aLibraryPath )
 {
     if( !m_cache || m_cache->m_lib_path != aLibraryPath ||
diff --git a/pcbnew/legacy_plugin.h b/pcbnew/legacy_plugin.h
index 4538388bd3..f8390b18a3 100644
--- a/pcbnew/legacy_plugin.h
+++ b/pcbnew/legacy_plugin.h
@@ -69,7 +69,7 @@ class LEGACY_PLUGIN : public PLUGIN
 
 public:
 
-    //-----<PLUGIN IMPLEMENTATION>----------------------------------------------
+    //-----<PLUGIN API>---------------------------------------------------------
 
     const wxString PluginName() const override
     {
@@ -93,9 +93,11 @@ public:
     bool FootprintLibDelete( const wxString& aLibraryPath,
                              const PROPERTIES* aProperties = NULL ) override;
 
+    long long GetLibraryTimestamp() const override;
+
     bool IsFootprintLibWritable( const wxString& aLibraryPath ) override;
 
-    //-----</PLUGIN IMPLEMENTATION>---------------------------------------------
+    //-----</PLUGIN API>--------------------------------------------------------
 
     typedef int     BIU;
 
diff --git a/pcbnew/pcad2kicadpcb_plugin/pcad_plugin.h b/pcbnew/pcad2kicadpcb_plugin/pcad_plugin.h
index c28f900698..138463825d 100644
--- a/pcbnew/pcad2kicadpcb_plugin/pcad_plugin.h
+++ b/pcbnew/pcad2kicadpcb_plugin/pcad_plugin.h
@@ -47,6 +47,12 @@ public:
 
     const wxString  GetFileExtension() const override;
 
+    long long GetLibraryTimestamp() const override
+    {
+        // No support for libraries....
+        return 0;
+    }
+
     // -----</PUBLIC PLUGIN API>-------------------------------------------------
 
     PCAD_PLUGIN();