Sending
Messages with Time-To-Be-Received (TTBR) set are stored in the queue table just like regular messages. The only difference is that the Expires column is set to the time (in UTC) when the message expires. The calculation of expiry time is done on the database level so that clock drifts between endpoints don't affect the TTBR.
Receiving
Messages with Time-To-Be-Received (TTBR) specified are pushed through the message pump, just like regular messages, regardless of if their TTBR already elapsed or not. The receive query compares the current UTC time in the database with the Expires column value and sets a flag indicating if a given message has expired. When the message is received from the database that flag is checked and if it is set, the message is dropped and not passed into the processing pipeline. Setting the flag on the database level prevents the TTBR feature from being affected by potential clock drift of machines running sender and receiver endpoints.
In addition to that, expired messages can be removed from the queue before the endpoints start up. In this case the transport removes all expired messages, in batches, using delete statements. Each batch is - by default - limited to 10,000 messages. The receiver is started when there are no messages to remove.
To enable start-up expired message purging use:
var transport = endpointConfiguration.UseTransport<SqlServerTransport>();
transport.PurgeExpiredMessagesOnStartup(purgeBatchSize: 5000);