I refer to ravi_9793's post above.May be your firewall is blocking.
If you have a firewall, put 129.6.15.29 in the trusted zone.(That is the IP address of time-b.nist. gov)
Most people who earlier were synchronising with time.windows.com successfully, but unable to synchronise now, should definitely be able to synchronise with time-b.nist.gov. That much is very certain.
In fact I have now changed the time server IP address to 129.6.15.29 in my Netgear DG834WG, and every time it boots it synchronises time without fail.(Earlier it was getting time from time-f.netgear.com)
If you still have problems it is for other geeks here like Choto cheeta, Krazy about technology, raptorz, infrareddude and their like to resolve