Garmin: Fix all-day events

As per the CalendarContract, the begin timestamp corresponds to the UTC
midnight-boundary of the start of the day.

- Remove the redundant logic to truncate the date to the start of day
- Remove the workaround for negative timezones (confirmed not workin
  with some EDT users)
- Ensure birthdays also respect the UTC boundary
- Offset the garmin timestamps by the UTC offset, ensuring they match
  midnight on the user's timezome (fixes all-day events offset by 1 day)

app/src/main/java/nodomain/freeyourgadget/gadgetbridge/externalevents/CalendarReceiver.java

@@ -28,16 +28,13 @@ import android.os.Handler;
 import android.provider.CalendarContract;
 import android.widget.Toast;
 
-import androidx.core.content.ContextCompat;
 import androidx.localbroadcastmanager.content.LocalBroadcastManager;
 
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
 import java.util.ArrayList;
-import java.util.Calendar;
 import java.util.Enumeration;
-import java.util.GregorianCalendar;
 import java.util.concurrent.TimeUnit;
 import java.util.Hashtable;
 import java.util.List;
@@ -234,15 +231,12 @@ public class CalendarReceiver extends ContentObserver {
                 calendarEventSpec.timestamp = calendarEvent.getBeginSeconds();
                 calendarEventSpec.durationInSeconds = calendarEvent.getDurationSeconds(); //FIXME: leads to problems right now
                 if (calendarEvent.isAllDay()) {
-                    //force the all day events to begin at midnight and last N whole days
+                    // As per the CalendarContract, for all-day events, the start timestamp is always in UTC
-                    Calendar c = GregorianCalendar.getInstance();
+                    // and corresponds to the midnight boundary
-                    int numDays = (int) TimeUnit.DAYS.convert(calendarEvent.getEnd() - calendarEvent.getBegin(),
+                    final int numDays = (int) TimeUnit.DAYS.convert(
-                            TimeUnit.MILLISECONDS);
+                            calendarEvent.getEnd() - calendarEvent.getBegin(),
-                    c.setTimeInMillis(calendarEvent.getBegin());
+                            TimeUnit.MILLISECONDS
-                    c.set(Calendar.HOUR_OF_DAY, 0);
+                    );
-                    //workaround for negative timezones
-                    if (c.getTimeZone().getRawOffset() < 0) c.add(Calendar.DAY_OF_MONTH, 1);
-                    calendarEventSpec.timestamp = (int) (c.getTimeInMillis() / 1000);
                     calendarEventSpec.durationInSeconds = 24 * 60 * 60 * numDays;
                 }
                 calendarEventSpec.description = calendarEvent.getDescription();

app/src/main/java/nodomain/freeyourgadget/gadgetbridge/service/devices/garmin/ProtocolBufferHandler.java

@@ -41,6 +41,7 @@ import nodomain.freeyourgadget.gadgetbridge.service.devices.garmin.messages.GFDI
 import nodomain.freeyourgadget.gadgetbridge.service.devices.garmin.messages.ProtobufMessage;
 import nodomain.freeyourgadget.gadgetbridge.service.devices.garmin.messages.status.ProtobufStatusMessage;
 import nodomain.freeyourgadget.gadgetbridge.service.devices.pebble.webview.CurrentPosition;
+import nodomain.freeyourgadget.gadgetbridge.util.DateTimeUtils;
 import nodomain.freeyourgadget.gadgetbridge.util.GB;
 import nodomain.freeyourgadget.gadgetbridge.util.calendar.CalendarEvent;
 import nodomain.freeyourgadget.gadgetbridge.util.calendar.CalendarManager;
@@ -212,11 +213,25 @@ public class ProtocolBufferHandler implements MessageHandler {
                     break;
                 }
 
+                final int startDateSeconds;
+                final int endDateSeconds;
+
+                if (mEvt.isAllDay()) {
+                    // For all-day events, garmin expects the start and end date to match the midnight boundaries
+                    // in the user's timezone. However, the calendar event will have them in the UTC timezone,
+                    // so we need to convert it
+                    startDateSeconds = (int) (DateTimeUtils.utcDateTimeToLocal(mEvt.getBegin()) / 1000);
+                    endDateSeconds = (int) (DateTimeUtils.utcDateTimeToLocal(mEvt.getEnd()) / 1000);
+                } else {
+                    startDateSeconds = mEvt.getBeginSeconds();
+                    endDateSeconds = mEvt.getEndSeconds();
+                }
+
                 final GdiCalendarService.CalendarService.CalendarEvent.Builder event = GdiCalendarService.CalendarService.CalendarEvent.newBuilder()
                         .setTitle(mEvt.getTitle().substring(0, Math.min(mEvt.getTitle().length(), calendarServiceRequest.getMaxTitleLength())))
                         .setAllDay(mEvt.isAllDay())
-                        .setStartDate(mEvt.getBeginSeconds())
+                        .setStartDate(startDateSeconds)
-                        .setEndDate(mEvt.getEndSeconds());
+                        .setEndDate(endDateSeconds);
 
                 if (calendarServiceRequest.getIncludeLocation() && mEvt.getLocation() != null) {
                     event.setLocation(mEvt.getLocation().substring(0, Math.min(mEvt.getLocation().length(), calendarServiceRequest.getMaxLocationLength())));

app/src/main/java/nodomain/freeyourgadget/gadgetbridge/util/DateTimeUtils.java

@@ -140,8 +140,8 @@ public class DateTimeUtils {
         return ret;
     }
 
-    public static Date dayStart(final LocalDate date) {
+    public static Date dayStartUtc(final LocalDate date) {
-        final Calendar calendar = Calendar.getInstance();
+        final Calendar calendar = Calendar.getInstance(TimeZone.getTimeZone("UTC"));
         calendar.set(Calendar.YEAR, date.getYear());
         calendar.set(Calendar.MONTH, date.getMonthValue() - 1);
         calendar.set(Calendar.DAY_OF_MONTH, date.getDayOfMonth());
@@ -152,6 +152,20 @@ public class DateTimeUtils {
         return calendar.getTime();
     }
 
+    public static long utcDateTimeToLocal(final long timestamp) {
+        final Calendar utcCalendar = Calendar.getInstance(TimeZone.getTimeZone("UTC"));
+        utcCalendar.setTimeInMillis(timestamp);
+        final Calendar localCalendar = Calendar.getInstance(TimeZone.getDefault());
+        localCalendar.set(Calendar.YEAR, utcCalendar.get(Calendar.YEAR));
+        localCalendar.set(Calendar.MONTH, utcCalendar.get(Calendar.MONTH));
+        localCalendar.set(Calendar.DAY_OF_MONTH, utcCalendar.get(Calendar.DAY_OF_MONTH));
+        localCalendar.set(Calendar.HOUR_OF_DAY, utcCalendar.get(Calendar.HOUR_OF_DAY));
+        localCalendar.set(Calendar.MINUTE, utcCalendar.get(Calendar.MINUTE));
+        localCalendar.set(Calendar.SECOND, utcCalendar.get(Calendar.SECOND));
+        localCalendar.set(Calendar.MILLISECOND, utcCalendar.get(Calendar.MILLISECOND));
+        return localCalendar.getTimeInMillis();
+    }
+
     public static Date dayEnd(final Date date) {
         final Calendar calendar = Calendar.getInstance();
         calendar.setTime(date);

app/src/main/java/nodomain/freeyourgadget/gadgetbridge/util/calendar/CalendarManager.java

@@ -35,11 +35,13 @@ import java.time.format.DateTimeParseException;
 import java.util.ArrayList;
 import java.util.Calendar;
 import java.util.Comparator;
+import java.util.Date;
 import java.util.GregorianCalendar;
 import java.util.HashSet;
 import java.util.LinkedList;
 import java.util.List;
 import java.util.Set;
+import java.util.TimeZone;
 
 import nodomain.freeyourgadget.gadgetbridge.GBApplication;
 import nodomain.freeyourgadget.gadgetbridge.R;
@@ -212,9 +214,13 @@ public class CalendarManager {
                     continue;
                 }
 
+                // Follow the same logic as CalendarContract - all day events have the start
+                // timestamp at the UTC midnight boundary
+                final long startTimestampUtc = DateTimeUtils.dayStartUtc(birthday).getTime();
+
                 birthdays.add(new CalendarEvent(
-                        DateTimeUtils.dayStart(birthday).getTime(),
+                        startTimestampUtc,
-                        DateTimeUtils.dayStart(birthday).getTime() + 86400000L - 1L,
+                        startTimestampUtc + 86400000L - 1L,
                         contactId.hashCode(),
                         mContext.getString(R.string.contact_birthday, displayName),
                         null,