updated docs

dlib/dir_nav/dir_nav_kernel_1.h

@@ -40,11 +40,13 @@ namespace dlib
                 state.name        == name()
                 state.full_name   == full_name()
                 state.file_size   == size()
+                state.last_modified == last_modified()
 
             CONVENTION
                 state.name        == name()
                 state.full_name   == full_name()
                 state.file_size   == size()
+                state.last_modified == last_modified()
 
         !*/
 

dlib/dir_nav/dir_nav_kernel_2.h

@@ -49,11 +49,13 @@ namespace dlib
                 state.name        == name()
                 state.full_name   == full_name()
                 state.file_size   == size()
+                state.last_modified == last_modified()
 
             CONVENTION
                 state.name        == name()
                 state.full_name   == full_name()
                 state.file_size   == size()
+                state.last_modified == last_modified()
 
         !*/
 

dlib/dir_nav/dir_nav_kernel_abstract.h

@@ -7,6 +7,7 @@
 #include <vector>
 #include "../uintn.h"
 #include "../algs.h"
+#include <chrono>
 
 namespace dlib
 {
@@ -139,6 +140,13 @@ namespace dlib
                 - returns the size of this file in bytes.
         !*/
 
+        std::chrono::time_point<std::chrono::system_clock> last_modified (
+        ) const;
+        /*!
+            ensures
+                - returns the time the file was last modified.
+        !*/
+
         operator std::string (
         ) const; 
         /*!