Fix issue with rendering of group update timestamps.

TIL SimpleDateFormat is not thread safe.
Across instances.

God forgive them, for they know not what they did.

app/src/main/java/org/thoughtcrime/securesms/util/DateUtils.java

@@ -40,9 +40,9 @@ import java.util.concurrent.TimeUnit;
 public class DateUtils extends android.text.format.DateUtils {
 
   @SuppressWarnings("unused")
-  private static final String           TAG                = DateUtils.class.getSimpleName();
-  private static final SimpleDateFormat DATE_FORMAT        = new SimpleDateFormat("yyyyMMdd");
-  private static final SimpleDateFormat BRIEF_EXACT_FORMAT = new SimpleDateFormat();
+  private static final String                        TAG                = DateUtils.class.getSimpleName();
+  private static final ThreadLocal<SimpleDateFormat> DATE_FORMAT        = new ThreadLocal<>();
+  private static final ThreadLocal<SimpleDateFormat> BRIEF_EXACT_FORMAT = new ThreadLocal<>();
 
   private static boolean isWithin(final long millis, final long span, final TimeUnit unit) {
     return System.currentTimeMillis() - millis <= unit.toMillis(span);
@@ -166,7 +166,10 @@ public class DateUtils extends android.text.format.DateUtils {
   }
 
   public static boolean isSameDay(long t1, long t2) {
-    return DATE_FORMAT.format(new Date(t1)).equals(DATE_FORMAT.format(new Date(t2)));
+    String d1 = getDateFormat().format(new Date(t1));
+    String d2 = getDateFormat().format(new Date(t2));
+
+    return d1.equals(d2);
   }
 
   public static boolean isSameExtendedRelativeTimestamp(@NonNull Context context, @NonNull Locale locale, long t1, long t2) {
@@ -214,4 +217,28 @@ public class DateUtils extends android.text.format.DateUtils {
       return -1;
     }
   }
+
+  @SuppressLint("SimpleDateFormat")
+  private static SimpleDateFormat getDateFormat() {
+    SimpleDateFormat format = DATE_FORMAT.get();
+
+    if (format == null) {
+      format = new SimpleDateFormat("yyyyMMdd");
+      DATE_FORMAT.set(format);
+    }
+
+    return format;
+  }
+
+  @SuppressLint("SimpleDateFormat")
+  private static SimpleDateFormat getBriefExactFormat() {
+    SimpleDateFormat format = BRIEF_EXACT_FORMAT.get();
+
+    if (format == null) {
+      format = new SimpleDateFormat();
+      BRIEF_EXACT_FORMAT.set(format);
+    }
+
+    return format;
+  }
 }