Add the time servers to "Source time servers queue":
If you use predefined source time servers, please press the "Predefined source time servers" button to open the "Predefined source time servers" window, highlight some source time servers, press the "Add to synchronization queue" button, and then press the "Close" button to close the window.
If you don't use predefined source time servers, please press the "New..." button to create new time server definition in "Source time servers queue".
Check source time servers in "Source time servers queue".
Set the "Automatic synchronization" options.
Set the "Retry if synchronization failed" options.