18. An apparatus according to claim 1 , wherein condition (b) is satisfied.
18. An apparatus according to claim 1 , wherein condition (b) is satisfied. 22. An apparatus according to claim 18 , wherein the algorithm different from the first chronological algorithm is such that when executed by the proxy server apparatus and when executed by the client device, the algorithm different from the first chronological algorithm returns a same chronological result in both cases irrespective of clocks in the proxy server apparatus and the client device being imperfectly synchronized.