WebService response caching

[Zürcher See]

I'm try to cache the response of a webmethod for about 1 minute, but it
doesn't work.
When I reload the page in the browser (http://localhost/ws.asmx/Clock), the
time has changed.
By caching the webmethod for 1 minute, the response should change every
minute, not every request?!?

[WebMethod(false,System.EnterpriseServices.Transact ionOption.Disabled,60)]
public string Clock(){return DateTime.Now.ToString();}