GPX: Fix parsing of timestamps without timezone

Although these are non-conforming, there are a lot of gpx files online
that do not contain a timezone and fail to be parsed.

app/src/main/java/nodomain/freeyourgadget/gadgetbridge/util/gpx/GpxParser.java

@@ -160,6 +160,10 @@ public class GpxParser {
         if (text == null || text.length() < 1) {
             return defaultDate;
         }
+        // If no timezone indicator is present, assume UTC
+        if (!text.endsWith("Z") && !text.contains("+") && !(text.length() > 19 && text.charAt(19) == '-')) {
+            return ISO8601Utils.parse(text + "Z", new ParsePosition(0));
+        }
         return ISO8601Utils.parse(text, new ParsePosition(0));
     }
 

app/src/test/java/nodomain/freeyourgadget/gadgetbridge/util/gpx/GPXParserTest.java

@@ -201,6 +201,31 @@ public class GPXParserTest extends TestBase {
         Assert.assertEquals(trackpoint.getVdop(), activityPoint.getLocation().getVdop(), 0.0);
     }
 
+    @Test
+    public void shouldParseDatetimeVariants() throws IOException, GpxParseException {
+        // Validates parsing of timestamps with no timezone (treated as UTC), explicit UTC (Z),
+        // positive offset, and negative offset.
+        try (final InputStream inputStream = getClass().getResourceAsStream("/gpx-parser-test-datetime.gpx")) {
+            final GpxParser gpxParser = new GpxParser(inputStream);
+            final GpxFile gpxFile = gpxParser.getGpxFile();
+
+            // Metadata time: No timezone indicator: should be treated as UTC
+            Assert.assertEquals(Date.from(Instant.parse("2026-05-23T19:31:00Z")), gpxFile.getTime());
+
+            final List<GpxTrackPoint> points = gpxFile.getPoints();
+            Assert.assertEquals(4, points.size());
+
+            // No timezone: treated as UTC
+            Assert.assertEquals(Date.from(Instant.parse("2026-05-23T19:31:24Z")), points.get(0).getTime());
+            // Explicit UTC (Z)
+            Assert.assertEquals(Date.from(Instant.parse("2026-05-23T19:32:00Z")), points.get(1).getTime());
+            // Positive offset (+02:00): 21:33 local = 19:33 UTC
+            Assert.assertEquals(Date.from(Instant.parse("2026-05-23T19:33:00Z")), points.get(2).getTime());
+            // Negative offset (-07:00): 12:34 local = 19:34 UTC
+            Assert.assertEquals(Date.from(Instant.parse("2026-05-23T19:34:00Z")), points.get(3).getTime());
+        }
+    }
+
     // test import of old GPX v1.0
     @Test
     public void TestGpxImportOld() throws Exception {

app/src/test/resources/gpx-parser-test-datetime.gpx

@@ -0,0 +1,35 @@
+<?xml version='1.0' encoding='UTF-8' standalone='yes'?>
+<gpx version="1.1" creator="NonConformingDevice"
+    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+    xmlns:gpxtpx="http://www.garmin.com/xmlschemas/TrackPointExtension/v1"
+    xmlns="http://www.topografix.com/GPX/1/1">
+    <metadata>
+        <name>Test Timezone Variants</name>
+        <!-- No timezone indicator: should be treated as UTC -->
+        <time>2026-05-23T19:31:00</time>
+    </metadata>
+    <trk>
+        <trkseg>
+            <!-- No timezone: treated as UTC -->
+            <trkpt lon="9.7393798828125" lat="47.5048828125">
+                <ele>440.0</ele>
+                <time>2026-05-23T19:31:24</time>
+            </trkpt>
+            <!-- Explicit UTC (Z) -->
+            <trkpt lon="9.7394798828125" lat="47.5058828125">
+                <ele>441.0</ele>
+                <time>2026-05-23T19:32:00Z</time>
+            </trkpt>
+            <!-- Positive offset (+02:00): 21:33 local = 19:33 UTC -->
+            <trkpt lon="9.7395798828125" lat="47.5068828125">
+                <ele>442.0</ele>
+                <time>2026-05-23T21:33:00+02:00</time>
+            </trkpt>
+            <!-- Negative offset (-07:00): 12:34 local = 19:34 UTC -->
+            <trkpt lon="9.7396798828125" lat="47.5078828125">
+                <ele>443.0</ele>
+                <time>2026-05-23T12:34:00-07:00</time>
+            </trkpt>
+        </trkseg>
+    </trk>
+</gpx>