WEEKDAY CALCULATION DEPENDING ON HOURS

Hi,

I have a table with job start times. From the job start times I want to calculate weekdays. If the hour of the job start time > 0 and < 6 then the weekday should be one day earlier (we have shifts from 6AM to 6AM the next morning).

Now I have the following calculation, however I for HOUR calculation and weekday calculation, I cannot select the relating start time. How should I solve this?