At last it seems the Tor network supports natively time queries which makes it the ideal way to check time for sdwdate.
Advantages:
*Attack surface reduced to that of the Tor daemon
*No need to worry about miantining lists of active onions or the upcoming deprecation of v2 addresses.
https://blog.torproject.org/new-release-tor-0348-also-other-stable-updates-02917-03212-and-03310
Minor features (control port):
Introduce GETINFO “current-time/{local,utc}” to return the local and UTC times respectively in ISO format. This helps a controller like Tor Browser detect a time-related error. Closes ticket 25511 (https://bugs.torproject.org/25511). Patch by Neel Chauhan.