Index: trunk/src/org/openstreetmap/josm/io/GpxWriter.java
===================================================================
--- trunk/src/org/openstreetmap/josm/io/GpxWriter.java	(revision 17721)
+++ trunk/src/org/openstreetmap/josm/io/GpxWriter.java	(revision 17722)
@@ -9,4 +9,5 @@
 import java.io.PrintWriter;
 import java.nio.charset.StandardCharsets;
+import java.time.Instant;
 import java.util.ArrayList;
 import java.util.Collection;
@@ -170,5 +171,7 @@
                     Object val = obj.get(key);
                     if (val instanceof Date) {
-                        simpleTag(key, DateUtils.fromDate((Date) val));
+                        throw new IllegalStateException();
+                    } else if (val instanceof Instant) {
+                        simpleTag(key, String.valueOf(val));
                     } else if (val instanceof Number) {
                         simpleTag(key, val.toString());