Write out timestamp values with full precision.

Fixes #31

Author: airbreather

doc/release-notes.md

@@ -1,7 +1,8 @@
 # NetTopologySuite.IO.GPX Release Notes
 
 ## [0.5.0](https://github.com/NetTopologySuite/NetTopologySuite.IO.GPX/milestone/10)
-*Nothing yet...*
+- `GpxWriterSettings.TimeZoneInfo` is no longer completely ignored ([#30](https://github.com/NetTopologySuite/NetTopologySuite.IO.GPX/issues/30)).
+- Fix an issue where written timestamps were being rounded to the nearest second instead of preserving as many significant digits as `DateTime` allows ([#31](https://github.com/NetTopologySuite/NetTopologySuite.IO.GPX/issues/31)).
 
 ## [0.4.0](https://github.com/NetTopologySuite/NetTopologySuite.IO.GPX/milestone/7)
 - `GpxReaderSettings` now has an `IgnoreBadDateTime` property to use for ignoring a timestamp value that we cannot parse as an instance of the `DateTime` struct, to work around a `0000-00-00T00:00:00Z` coming from CompeGPS ([#29](https://github.com/NetTopologySuite/NetTopologySuite.IO.GPX/issues/29)).

src/NetTopologySuite.IO.GPX/Helpers.cs

@@ -329,7 +329,10 @@ public static void WriteOptionalGpxElementValue(this XmlWriter writer, string lo
         {
             if (valueUtc.HasValue)
             {
-                const string Format = "yyyy-MM-ddTHH:mm:ssK";
+                // Format string is hardcoded, instead of allowing the user to override it, to
+                // ensure that we always write out timestamps that are valid according to the
+                // published XML schema.
+                const string Format = "yyyy-MM-ddTHH:mm:ss.FFFFFFFK";
 
                 string text;
                 if (timeZoneInfo == TimeZoneInfo.Utc)

tests/NetTopologySuite.IO.GPX.Tests/GpxFileTests.cs

@@ -323,5 +323,30 @@ public void TimestampsShouldBeWrittenInGivenTimeZone()
             Assert.Contains("1234-05-06T10:38:09+03:30", text);
             Assert.Contains("5432-10-10T14:52:33+03:30", text);
         }
+
+        [Fact]
+        [Regression]
+        [GitHubIssue(31)]
+        public void TimestampsShouldPreserveFractionalSecondsWithinDefinedPrecision()
+        {
+            const string GpxText = @"
+<gpx xmlns='http://www.topografix.com/GPX/1/1' version='1.1' creator='airbreather'>
+    <metadata>
+        <time>1234-05-06T07:08:09.7654321</time>
+    </metadata>
+    <wpt lat='0.1' lon='2.3'>
+        <time>5432-10-10T11:22:33.87654321</time>
+    </wpt>
+    <wpt lat='4.5' lon='6.7'>
+        <time>1111-11-11T11:11:11.12345</time>
+    </wpt>
+</gpx>
+";
+            string text = GpxFile.Parse(GpxText, null). BuildString(null);
+
+            Assert.Contains("1234-05-06T07:08:09.7654321Z", text);
+            Assert.Contains("5432-10-10T11:22:33.8765432Z", text); // DateTime resolution is 100ns, so the value gets rounded to 7 digits
+            Assert.Contains("1111-11-11T11:11:11.12345Z", text); // don't output extra zeroes
+        }
     }
 }