diff --git a/static/js/modes/weather-satellite.js b/static/js/modes/weather-satellite.js
index 0896690..fc0fdb8 100644
--- a/static/js/modes/weather-satellite.js
+++ b/static/js/modes/weather-satellite.js
@@ -461,6 +461,26 @@ const WeatherSat = (function() {
         return `${m}:${s.toString().padStart(2, '0')}`;
     }
 
+    /**
+     * Parse pass timestamps, accepting legacy malformed UTC strings (+00:00Z).
+     */
+    function parsePassDate(value) {
+        if (!value || typeof value !== 'string') return null;
+
+        let parsed = new Date(value);
+        if (!Number.isNaN(parsed.getTime())) {
+            return parsed;
+        }
+
+        // Backward-compatible cleanup for accidentally double-suffixed UTC timestamps.
+        parsed = new Date(value.replace(/\+00:00Z$/, 'Z'));
+        if (!Number.isNaN(parsed.getTime())) {
+            return parsed;
+        }
+
+        return null;
+    }
+
     /**
      * Load pass predictions (with trajectory + ground track)
      */
@@ -553,13 +573,15 @@ const WeatherSat = (function() {
             const modeClass = pass.mode === 'APT' ? 'apt' : 'lrpt';
             const timeStr = pass.startTime || '--';
             const now = new Date();
-            const passStart = new Date(pass.startTimeISO);
-            const diffMs = passStart - now;
-            const diffMins = Math.floor(diffMs / 60000);
+            const passStart = parsePassDate(pass.startTimeISO);
+            const diffMs = passStart ? passStart - now : NaN;
+            const diffMins = Number.isFinite(diffMs) ? Math.floor(diffMs / 60000) : NaN;
             const isSelected = idx === selectedPassIndex;
 
-            let countdown = '';
-            if (diffMs < 0) {
+            let countdown = '--';
+            if (!Number.isFinite(diffMs)) {
+                countdown = '--';
+            } else if (diffMs < 0) {
                 countdown = 'NOW';
             } else if (diffMins < 60) {
                 countdown = `in ${diffMins}m`;
@@ -828,8 +850,11 @@ const WeatherSat = (function() {
         let isActive = false;
 
         for (const pass of passes) {
-            const start = new Date(pass.startTimeISO);
-            const end = new Date(pass.endTimeISO);
+            const start = parsePassDate(pass.startTimeISO);
+            const end = parsePassDate(pass.endTimeISO);
+            if (!start || !end) {
+                continue;
+            }
             if (end > now) {
                 nextPass = pass;
                 isActive = start <= now;
@@ -858,7 +883,19 @@ const WeatherSat = (function() {
             return;
         }
 
-        const target = new Date(nextPass.startTimeISO);
+        const target = parsePassDate(nextPass.startTimeISO);
+        if (!target) {
+            if (daysEl) daysEl.textContent = '--';
+            if (hoursEl) hoursEl.textContent = '--';
+            if (minsEl) minsEl.textContent = '--';
+            if (secsEl) secsEl.textContent = '--';
+            if (satEl) satEl.textContent = '--';
+            if (detailEl) detailEl.textContent = 'Invalid pass time';
+            if (boxes) boxes.querySelectorAll('.wxsat-countdown-box').forEach(b => {
+                b.classList.remove('imminent', 'active');
+            });
+            return;
+        }
         let diffMs = target - now;
 
         if (isActive) {
@@ -918,8 +955,9 @@ const WeatherSat = (function() {
         const dayMs = 24 * 60 * 60 * 1000;
 
         passList.forEach((pass, idx) => {
-            const start = new Date(pass.startTimeISO);
-            const end = new Date(pass.endTimeISO);
+            const start = parsePassDate(pass.startTimeISO);
+            const end = parsePassDate(pass.endTimeISO);
+            if (!start || !end) return;
 
             const startPct = Math.max(0, Math.min(100, ((start - dayStart) / dayMs) * 100));
             const endPct = Math.max(0, Math.min(100, ((end - dayStart) / dayMs) * 100));
diff --git a/tests/test_weather_sat_predict.py b/tests/test_weather_sat_predict.py
index 97c2e29..50220f2 100644
--- a/tests/test_weather_sat_predict.py
+++ b/tests/test_weather_sat_predict.py
@@ -7,10 +7,10 @@ and ground track generation.
 from __future__ import annotations
 
 from datetime import datetime, timezone, timedelta
-from unittest.mock import patch, MagicMock
+from unittest.mock import MagicMock, patch
 import pytest
 
-from utils.weather_sat_predict import predict_passes
+from utils.weather_sat_predict import _format_utc_iso, predict_passes
 
 
 class TestPredictPasses:
@@ -673,3 +673,20 @@ class TestPassDataStructure:
         with patch.dict('sys.modules', {'skyfield': None, 'skyfield.api': None}):
             with pytest.raises((ImportError, AttributeError)):
                 predict_passes(lat=51.5, lon=-0.1)
+
+
+class TestTimestampFormatting:
+    """Tests for UTC timestamp serialization helpers."""
+
+    def test_format_utc_iso_from_aware_datetime(self):
+        """Aware UTC datetimes should not get a duplicate UTC suffix."""
+        dt = datetime(2024, 1, 1, 12, 0, 0, tzinfo=timezone.utc)
+        value = _format_utc_iso(dt)
+        assert value == '2024-01-01T12:00:00Z'
+        assert '+00:00Z' not in value
+
+    def test_format_utc_iso_from_naive_datetime(self):
+        """Naive datetimes should be treated as UTC and serialized consistently."""
+        dt = datetime(2024, 1, 1, 12, 0, 0)
+        value = _format_utc_iso(dt)
+        assert value == '2024-01-01T12:00:00Z'
diff --git a/utils/weather_sat_predict.py b/utils/weather_sat_predict.py
index 88dfd32..de8f538 100644
--- a/utils/weather_sat_predict.py
+++ b/utils/weather_sat_predict.py
@@ -14,6 +14,15 @@ from utils.weather_sat import WEATHER_SATELLITES
 logger = get_logger('intercept.weather_sat_predict')
 
 
+def _format_utc_iso(dt: datetime.datetime) -> str:
+    """Return an ISO8601 UTC timestamp with a single timezone designator."""
+    if dt.tzinfo is None:
+        dt = dt.replace(tzinfo=datetime.timezone.utc)
+    else:
+        dt = dt.astimezone(datetime.timezone.utc)
+    return dt.isoformat().replace('+00:00', 'Z')
+
+
 def predict_passes(
     lat: float,
     lon: float,
@@ -38,8 +47,9 @@ def predict_passes(
     Raises:
         ImportError: If skyfield is not installed.
     """
-    from skyfield.api import load, wgs84, EarthSatellite
     from skyfield.almanac import find_discrete
+    from skyfield.api import EarthSatellite, load, wgs84
+
     from data.satellites import TLE_SATELLITES
 
     # Use live TLE cache from satellite module if available (refreshed from CelesTrak)
@@ -100,8 +110,10 @@ def predict_passes(
                     i += 1
                     continue
 
+                rise_dt = rise_time.utc_datetime()
+                set_dt = set_time.utc_datetime()
                 duration_seconds = (
-                    set_time.utc_datetime() - rise_time.utc_datetime()
+                    set_dt - rise_dt
                 ).total_seconds()
                 duration_minutes = round(duration_seconds / 60, 1)
 
@@ -141,14 +153,14 @@ def predict_passes(
                 _, set_az, _ = set_topo.altaz()
 
                 pass_data: dict[str, Any] = {
-                    'id': f"{sat_key}_{rise_time.utc_datetime().strftime('%Y%m%d%H%M%S')}",
+                    'id': f"{sat_key}_{rise_dt.strftime('%Y%m%d%H%M%S')}",
                     'satellite': sat_key,
                     'name': sat_info['name'],
                     'frequency': sat_info['frequency'],
                     'mode': sat_info['mode'],
-                    'startTime': rise_time.utc_datetime().strftime('%Y-%m-%d %H:%M UTC'),
-                    'startTimeISO': rise_time.utc_datetime().isoformat() + 'Z',
-                    'endTimeISO': set_time.utc_datetime().isoformat() + 'Z',
+                    'startTime': rise_dt.strftime('%Y-%m-%d %H:%M UTC'),
+                    'startTimeISO': _format_utc_iso(rise_dt),
+                    'endTimeISO': _format_utc_iso(set_dt),
                     'maxEl': round(max_el, 1),
                     'maxElAz': round(max_el_az, 1),
                     'riseAz': round(rise_az.degrees, 1),