Change date by time

I believe I understand this as for any times before 6am, the new date column should show the previous date, for times of 6am or later it should show the 2nd Weight date. Because this is a static property of the 2nd weight, and the 6am changeover time doesn't change, I suggest doing this in PowerQuery. 

Here's an expression to do this with a custom column in PowerQuery:

if Time.Hour([2nd Weight Time]) >= 6 then [2nd Weight Date] else Date.AddDays([2nd Weight Date], -1)

Here's the equivalent in DAX for a calculated column, but this would be less performant, so try the above first: