Display activity date in local time (using the time zone that is stored with the activity), not in UTC (#4)

* Display timestamps using the timezone that is stored at the activity (fix 'new Date()' invocation)

* Display timestamps using the timezone that is stored at the activity (relative date in timeline views)

* Use correct timezone for auto-generated activity title

---------

Co-authored-by: Niklas Deutschmann <sonstharmlos@noreply.codeberg.org>

src/main/java/net/javahippie/fitpub/service/ActivityFileService.java

@@ -386,7 +386,8 @@ public class ActivityFileService {
             activityTitle = parsedData.getTitle();
         } else {
             // Generate title if not provided
-            activityTitle = ActivityFormatter.generateActivityTitle(parsedData.getStartTime(), parsedData.getActivityType());
+            activityTitle = ActivityFormatter.generateActivityTitle(parsedData.getStartTime(), parsedData.getTimezone(),
+                    parsedData.getActivityType());
         }
 
         // Default to PUBLIC if visibility not specified

src/main/java/net/javahippie/fitpub/service/FitFileService.java

@@ -24,7 +24,6 @@ import org.springframework.web.multipart.MultipartFile;
 
 import java.io.IOException;
 import java.math.BigDecimal;
-import java.time.LocalDateTime;
 import java.util.List;
 import java.util.UUID;
 
@@ -257,7 +256,7 @@ public class FitFileService {
     private String generateTitle(ParsedActivityData parsedData) {
         return ActivityFormatter.generateActivityTitle(
             parsedData.getStartTime(),
-            parsedData.getActivityType()
+                parsedData.getTimezone(), parsedData.getActivityType()
         );
     }
 

src/main/java/net/javahippie/fitpub/util/ActivityFormatter.java

@@ -1,13 +1,14 @@
 package net.javahippie.fitpub.util;
 
+import lombok.extern.slf4j.Slf4j;
 import net.javahippie.fitpub.model.entity.Activity;
 
-import java.time.LocalDateTime;
-import java.time.LocalTime;
+import java.time.*;
 
 /**
  * Utility class for formatting activity-related data for display.
  */
+@Slf4j
 public class ActivityFormatter {
 
     /**
@@ -47,21 +48,25 @@ public class ActivityFormatter {
      * Generates a default activity title based on the time of day and activity type.
      * Format: "[Time of Day] [Activity Type]" (e.g., "Morning Run", "Evening Ride")
      *
-     * @param startedAt the activity start time
+     * @param startedAt    the activity start time
+     * @param timezone     the timezone ID of the activity
      * @param activityType the activity type
      * @return generated title
      */
-    public static String generateActivityTitle(LocalDateTime startedAt, Activity.ActivityType activityType) {
+    public static String generateActivityTitle(LocalDateTime startedAt, String timezone, Activity.ActivityType activityType) {
         if (startedAt == null || activityType == null) {
             return "Activity";
         }
 
-        String timeOfDay = getTimeOfDay(startedAt.toLocalTime());
+        LocalDateTime startedAtLocal = getUtcDateTimeInZone(startedAt, timezone);
+        String timeOfDay = getTimeOfDay(startedAtLocal.toLocalTime());
         String formattedType = formatActivityType(activityType);
 
         return timeOfDay + " " + formattedType;
     }
 
+
+
     /**
      * Determines the time of day based on the hour.
      *
@@ -81,4 +86,25 @@ public class ActivityFormatter {
             return "Night";
         }
     }
+
+    /**
+     * Attempts to convert the given LocalDateTime (which is assumed to be UTC) into a LocalDateTime in the given
+     * timezone
+     *
+     * @param utcDateTime The original date and time (UTC)
+     * @param timezone A timezone ID
+     * @return The original date and time adjusted to the timezone, if the zone ID could be parsed. The original date
+     * and time otherwise
+     *
+     */
+    private static LocalDateTime getUtcDateTimeInZone(LocalDateTime utcDateTime, String timezone) {
+        try {
+            return utcDateTime.atZone(ZoneOffset.UTC)
+                    .withZoneSameInstant(ZoneId.of(timezone))
+                    .toLocalDateTime();
+        } catch (DateTimeException e) {
+            log.warn("Invalid time zone ID: {}", timezone);
+            return utcDateTime;
+        }
+    }
 }

src/main/resources/static/js/fitpub.js

@@ -434,7 +434,7 @@ function formatDateTimeWithTimezone(timestamp, timezone, options = {}) {
 
     // Parse the timestamp - backend sends LocalDateTime without 'Z'
     // We need to interpret it in the specified timezone
-    const date = new Date(timestamp);
+    const date = new Date(ensureUTC(timestamp));
 
     // Default options for date/time display
     const defaultOptions = {
@@ -473,6 +473,17 @@ function formatDateWithTimezone(timestamp, timezone) {
     });
 }
 
+/**
+ * Ensures that a timestamp will be interpreted as UTC by new Date()
+ * See https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date (Date time string format)
+ *
+ * @param {string} timestamp - ISO timestamp or LocalDateTime string
+ * @returns {string} The input string, but with a trailing 'Z'
+ */
+function ensureUTC(timestamp) {
+    return timestamp.endsWith('Z') ? timestamp : timestamp + 'Z';
+}
+
 // Make functions available globally for inline scripts
 window.FitPub = {
     createActivityMap,
@@ -482,5 +493,6 @@ window.FitPub = {
     formatDistance,
     formatPace,
     formatDateTimeWithTimezone,
-    formatDateWithTimezone
+    formatDateWithTimezone,
+    ensureUTC
 };

src/main/resources/static/js/timeline.js

@@ -727,7 +727,7 @@ const FitPubTimeline = {
      * @returns {string} Time ago string
      */
     formatTimeAgo: function(timestamp) {
-        const date = new Date(timestamp);
+        const date = new Date(FitPub.ensureUTC(timestamp));
         const now = new Date();
         const secondsAgo = Math.floor((now - date) / 1000);