Quick answer:
Unfortunately, it is not possible.
Detailed answer:
Even if the cross-origin policy allowed for it, obtaining NTP directly through Ajax without PHP (or a similar tool) acting as an intermediary is simply not feasible. One major reason is that time servers typically operate on UDP port 123, which Ajax cannot handle. Additionally, when Ajax sends a request to a server, it expects specific HTTP headers, status codes, and a response body in return. NTP does not conform to this structure, as it only sends a string response. Furthermore, HTML5 does not support raw socket connections.
However, one workaround using Ajax is examining the response headers, as many responses include a timestamp similar to this:
Date:Mon, 21 May 2012 15:30:58 GMT
By analyzing these headers, you can derive the current time.