diff --git a/HTTP-API.md b/HTTP-API.md
index 900b7ee..c474942 100644
--- a/HTTP-API.md
+++ b/HTTP-API.md
@@ -138,7 +138,7 @@ Theses authors are bind to the attributes the users choose (color and name).
 ## Session
 Sessions can be created between a group and an author. This allows an author to access more than one group. The sessionID will be set as a cookie to the client and is valid until a certain date. Only users with a valid session for this group, can access group pads. You can create a session after you authenticated the user at your web application, to give them access to the pads. You should save the sessionID of this session and delete it after the user logged out
 
-* **createSession(groupID, authorID, validUntil)** creates a new session <br><br>*Example returns:*
+* **createSession(groupID, authorID, validUntil)** creates a new session. validUntil is an unix timestamp in seconds <br><br>*Example returns:*
   * `{code: 0, message:"ok", data: {sessionID: "s.s8oes9dhwrvt0zif"}}`
   * `{code: 1, message:"groupID doesn't exist", data: null}`
   * `{code: 1, message:"authorID doesn't exist", data: null}`