Index: tests/natools-time_keys-tests.adb
==================================================================
--- tests/natools-time_keys-tests.adb
+++ tests/natools-time_keys-tests.adb
@@ -19,10 +19,16 @@
 
 package body Natools.Time_Keys.Tests is
 
    function Image (Date : Ada.Calendar.Time) return String;
 
+   procedure Key_Test
+     (Test : in out NT.Test;
+      Time : in Ada.Calendar.Time;
+      Expected_Key : in String;
+      Max_Sub_Second_Digits : in Natural);
+
    procedure Roundtrip_Test
      (Test : in out NT.Test;
       Time : in Ada.Calendar.Time;
       Expected_Key : in String);
 
@@ -36,10 +42,27 @@
       return Time_IO.RFC_3339.Image
         (Date => Date,
          Subsecond_Digits => Duration'Aft);
    end Image;
 
+
+   procedure Key_Test
+     (Test : in out NT.Test;
+      Time : in Ada.Calendar.Time;
+      Expected_Key : in String;
+      Max_Sub_Second_Digits : in Natural)
+   is
+      Generated_Key : constant String := To_Key (Time, Max_Sub_Second_Digits);
+   begin
+      if Generated_Key /= Expected_Key then
+         Test.Fail ("Generated key """ & Generated_Key
+           & """, expected """ & Expected_Key & '"');
+         Test.Info ("Time of generated key: "
+           & Image (To_Time (Generated_Key)));
+      end if;
+   end Key_Test;
+
 
    procedure Roundtrip_Test
      (Test : in out NT.Test;
       Time : in Ada.Calendar.Time;
       Expected_Key : in String)
@@ -67,10 +90,11 @@
    -------------------------
 
    procedure All_Tests (Report : in out NT.Reporter'Class) is
    begin
       Roundtrips (Report);
+      Subsecond_Rounding (Report);
    end All_Tests;
 
 
 
    ----------------------
@@ -110,6 +134,84 @@
          "_0CV");
    exception
       when Error : others => Test.Report_Exception (Error);
    end Roundtrips;
 
+
+   procedure Subsecond_Rounding (Report : in out NT.Reporter'Class) is
+      Test : NT.Test := Report.Item ("Overflow in subsecond rounding");
+   begin
+      if Duration'Small > 1.0 / 256.0 then
+         Test.Skip ("Not enough precision in Duration");
+         return;
+      end if;
+
+      Key_Test
+        (Test,
+         To_Time ("VV121231~V"),
+         "VV121231~",
+         2);
+
+      Key_Test
+        (Test,
+         To_Time ("VV121231~X"),
+         "VV121232",
+         2);
+
+      Key_Test
+        (Test,
+         To_Time ("VV124561~~V"),
+         "VV124561~~",
+         3);
+
+      Key_Test
+        (Test,
+         To_Time ("VV124561~~X"),
+         "VV124562",
+         3);
+
+      Key_Test
+        (Test,
+         Ada.Calendar.Formatting.Time_Of
+           (2015, 2, 2,  1,  1,  1, 255.0 / 256.0),
+         "VV22112",
+         1);
+
+      Key_Test
+        (Test,
+         Ada.Calendar.Formatting.Time_Of
+           (2015, 2, 2,  1,  58, 59, 255.0 / 256.0),
+         "VV221w",
+         1);
+
+      Key_Test
+        (Test,
+         Ada.Calendar.Formatting.Time_Of
+           (2015, 2, 2,  22,  59, 59, 255.0 / 256.0),
+         "VV22N",
+         1);
+
+      Key_Test
+        (Test,
+         Ada.Calendar.Formatting.Time_Of
+           (2015, 2, 28, 23, 59, 59, 255.0 / 256.0),
+         "VV31",
+         1);
+
+      Key_Test
+        (Test,
+         Ada.Calendar.Formatting.Time_Of
+           (2016, 2, 28, 23, 59, 59, 255.0 / 256.0),
+         "VW2T",
+         1);
+
+      Key_Test
+        (Test,
+         Ada.Calendar.Formatting.Time_Of
+           (2015, 12, 31, 23, 59, 59, 255.0 / 256.0),
+         "VW11",
+         1);
+   exception
+      when Error : others => Test.Report_Exception (Error);
+   end Subsecond_Rounding;
+
 end Natools.Time_Keys.Tests;

Index: tests/natools-time_keys-tests.ads
==================================================================
--- tests/natools-time_keys-tests.ads
+++ tests/natools-time_keys-tests.ads
@@ -21,7 +21,8 @@
    package NT renames Natools.Tests;
 
    procedure All_Tests (Report : in out NT.Reporter'Class);
 
    procedure Roundtrips (Report : in out NT.Reporter'Class);
+   procedure Subsecond_Rounding (Report : in out NT.Reporter'Class);
 
 end Natools.Time_Keys.Tests;