Hi Jim,
Currently, we don’t have support for hours in holiday property of Gantt Chart.
We logged this requirement as usability feature. You can track the status of this feature from below feedback link.
As we have already lined up some major features, we could not start its implementation immediately. We will include this support in any of our upcoming releases.
Please cast your vote to make it count. So, that we will prioritize the features for every release based on demands.
Regards,
Logeswari G