Fix timestamp of validation data

app/src/main/java/uk/org/openseizuredetector/SensorValidationActivity.java

@@ -529,10 +529,13 @@ public class SensorValidationActivity extends AppCompatActivity {
 
         double averageHr = currentHrValidCount > 0 ? currentHrSum / currentHrValidCount : -1.0;
         long timestampMillis = hrStartMillis + currentHrSecond * 1000L;
+        String timestampIso = timestampIsoLocal(timestampMillis);
         String timestamp = new SimpleDateFormat("HH:mm:ss", Locale.UK).format(new Date(timestampMillis));
 
         hrPoints.add(new HeartRatePoint(
                 hrPoints.size(),
+                timestampIso,
+                timestampMillis,
                 timestamp,
                 currentHrSecond,
                 averageHr,
@@ -605,9 +608,11 @@ public class SensorValidationActivity extends AppCompatActivity {
         File file = new File(getOutputDir(), "validation_hr_" + timestampForFilename() + ".csv");
         try {
             OutputStreamWriter writer = new OutputStreamWriter(new FileOutputStream(file));
-            writer.write("point_index,time_hhmmss,elapsed_seconds,avg_hr_bpm,valid,valid_readings_in_second,total_readings_in_second\n");
+            writer.write("point_index,timestamp_iso,epoch_ms,time_hhmmss,elapsed_seconds,avg_hr_bpm,valid,valid_readings_in_second,total_readings_in_second\n");
             for (HeartRatePoint point : hrPoints) {
                 writer.write(point.index + ","
+                        + point.timestampIso + ","
+                        + point.epochMillis + ","
                         + point.timestampHhMmSs + ","
                         + point.elapsedSecond + ","
                         + (point.valid ? String.format(Locale.UK, "%.3f", point.averageHr) : "") + ","
@@ -625,6 +630,8 @@ public class SensorValidationActivity extends AppCompatActivity {
 
     private static class HeartRatePoint {
         final int index;
+        final String timestampIso;
+        final long epochMillis;
         final String timestampHhMmSs;
         final int elapsedSecond;
         final double averageHr;
@@ -633,12 +640,16 @@ public class SensorValidationActivity extends AppCompatActivity {
         final int readingsInSecond;
 
         HeartRatePoint(int index,
+                       String timestampIso,
+                       long epochMillis,
                        String timestampHhMmSs,
                        int elapsedSecond,
                        double averageHr,
                        int validReadingsInSecond,
                        int readingsInSecond) {
             this.index = index;
+            this.timestampIso = timestampIso;
+            this.epochMillis = epochMillis;
             this.timestampHhMmSs = timestampHhMmSs;
             this.elapsedSecond = elapsedSecond;
             this.averageHr = averageHr;
@@ -661,6 +672,11 @@ public class SensorValidationActivity extends AppCompatActivity {
         return new SimpleDateFormat("yyyyMMdd_HHmmss", Locale.UK).format(new Date());
     }
 
+    private static String timestampIsoLocal(long millis) {
+        // ISO-like local timestamp with timezone offset, for example: 2026-05-05T10:24:25.000+0000
+        return new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss.SSSZ", Locale.UK).format(new Date(millis));
+    }
+
     private void saveRawCsv() {
         if (lastResult == null) return;
         File file = new File(getOutputDir(), "validation_raw_" + timestampForFilename() + ".csv");