Hello @Paul.Gerber,
Thank You for posting your query onto the Forum.
If you’re looking for a solution from the Power Query perspective then I absolutely agree with our expert @AntrikshSharma that merging the columns is the solution and then adding a separate column in the table that shows the differential time. But merging of columns can also cause an issue while doing the Time intelligence analysis since in that type of cases or scenarios it’s recommended to have date and time in two separate columns.
So couple of months back one of our forum member did posted a similar scenario and below is the link of that post provided for the reference purposes from where you can also download the PBIX file since in the current scenario there isn’t posted one.
Hoping you find this useful and helpful. 
Thanks and Warm Regards,
Harsh