The WebTimer service is traceable to UTC(AUS), the Australian realisation of Co-ordinated Universal Time (UTC). UTC(AUS) is the legal reference for time and frequency in Australia and is the output of a caesium atomic clock operated at the National Measurement Institute's Lindfield laboratory in Sydney.
The WebTimer server is directly synchronized by the 1 pulse-per-second output of UTC(AUS). Any residual difference between the server time and UTC(AUS) is measured and recorded; this difference is no more than a few tens of microseconds.
For a technical description of the traceability chain, including the uncertainty budget, please read this document.