It is possible to set a time limit for the maximum time between two consecutive messages of a client (option SetClientTimeLimit in the [Misc] section of the configuration file). If the time limit is exceeded without a message from the client, the server will issue a warning. The default is 86400 seconds (one day); specifying a value of 0 will switch off this option.

You may want to set ExportSeverity = mark (or any lower threshold) in the client configuration file in order to log timestamp ('heartbeat') messages to the server.