Must be one-time or strtotime date / time representation syntax

Why am I having this error when running the facebook api graphical code in C #?

(OAuthException - #100) Must be a unixtime or a date/time representation parseable by strtotime()

      

        
        
        
      

    

my graph Api Code:

https://graph.facebook.com/v2.1/687925031280154/feed?since=1391244171

      

        
        
        
      

    

but the same code returns the correct result when i use: developers.facebook.com/tools/explorer/