Extract util methods to separate classes

src/main/java/org/asamk/signal/util/DateUtils.java

@@ -0,0 +1,21 @@
+package org.asamk.signal.util;
+
+import java.text.DateFormat;
+import java.text.SimpleDateFormat;
+import java.util.Date;
+import java.util.TimeZone;
+
+public class DateUtils {
+
+    private static final TimeZone tzUTC = TimeZone.getTimeZone("UTC");
+
+    private DateUtils() {
+    }
+
+    public static String formatTimestamp(long timestamp) {
+        Date date = new Date(timestamp);
+        final DateFormat df = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss.SSS'Z'"); // Quoted "Z" to indicate UTC, no timezone offset
+        df.setTimeZone(tzUTC);
+        return timestamp + " (" + df.format(date) + ")";
+    }
+}