Make qmail dequeue only during certain time period per day

I am using qmail to send a large number of emails on behalf of many people. It varies between 400k and 600k emails per job. A php script queues up all of the messages, which takes a few hours. The messages then take about 24-48 hours to dequeue and get sent. The length of time is not a problem, but I do not like that emails are sent at odd times (like in the middle of the night, or at sunrise). **Is there a way I can tell qmail to only dequeue messages only during a certain time range?** For example, tell it to only empty the queue between 6AM and 8PM, then just let it "sleep" after 8PM until 6AM the next day to finish the queue. *Using CentOS 6.6 Final*