SDK-38 Improve the explanation of timestamps in documentation

README.md

@@ -33,7 +33,7 @@ The identification is performed asynchronously and there is no need to wait for
 
 You may track any event by calling the <code>track()</code> method on the Infinario instance. The <code>track()</code> method takes one mandatory and two optional arguments. First argument is <code>String type</code> which categorizes your event. This argument is <strong>required</strong>. You may choose any string you like.
 
-Next two arguments are <code>Map&lt;String, Object&gt; properties</code> and <code>Long timestamp</code>. Properties is a map which uses <code>String</code> keys and the value may be any <code>Object</code> which is serializable by <code>org.json.JSONObject</code> class. Properties can be used to attach any additional data to the event. Timestamp is standard UNIX timestamp in milliseconds and it can be used to mark the time of the event's occurence. The default timestamp is preset to the time of the tracking of the event.
+Next two arguments are <code>Map&lt;String, Object&gt; properties</code> and <code>Long timestamp</code>. Properties is a map which uses <code>String</code> keys and the value may be any <code>Object</code> which is serializable by <code>org.json.JSONObject</code> class. Properties can be used to attach any additional data to the event. Timestamp is in milliseconds since 1970-01-01 00:00:00 GMT and it can be used to mark the time of the event's occurence. The default timestamp is preset to the time of the tracking of the event.
 <pre><code>Map&lt;String, Object&gt; properties = new HashMap&lt;&gt;();
 properties.put("item_id", 45);
 Long timestamp = System.currentTimeMillis();

src/main/java/com/infinario/android/infinariosdk/Infinario.java

@@ -421,7 +421,7 @@ private Map<String, Object> mergeProperties(Map<String, Object> properties, long
      *
      * @param type       event's type
      * @param properties event's properties
-     * @param timestamp  event's timestamp in milliseconds
+     * @param timestamp  event's timestamp in milliseconds since 1970-01-01 00:00:00 GMT
      * @return success of the operation
      */
     public boolean track(String type, Map<String, Object> properties, Long timestamp) {
@@ -471,7 +471,7 @@ public boolean track(String type, Map<String, Object> properties) {
      * Tracks an event for a customer. Cannot be called prior to the identification.
      *
      * @param type event's type
-     * @param timestamp event's timestamp in milliseconds
+     * @param timestamp event's timestamp in milliseconds since 1970-01-01 00:00:00 GMT
      * @return success of the operation
      */
     public boolean track(String type, Long timestamp) {