Deduct Dates - Drop Down List

sure, something like =IF(cell where your priority is = "Low",N2-14,IF(cell where your priority is ="Medium",N2-30,IF(cell where your priority is ="High",N2-60,"")))
untested of course and where it says cell where your priority is would be the cell where your priority is and I'm using cell N2 as the example for your date cell.
substitute those as they are appropriate to your data.

well I don't know what formula you are using nor where your data is located nor what exactly your dates and text looks like.
the best thing is if you can upload a sample workbook of your data WITH expected results (nothing confidential or propiratory) by following the instructions to upload a workbook that are in the yellow banner at the top of this post. That is the best thing because otherwise I'm guessing (and I'm lazy and don't want to try to create a workbook based on your snapshot so I can take a snapshot to post).

imo, instead of using dropdown I'm prefered to use formula for this case.
by create range of words and deduct day for sample range G3:H5

G3=Low
H3=14
G4=Medium
H4=30
G5=High
H5=60

and I will use this formula to calculate for sample at row 9

=IF(F9>0,IFError(F9-Vlookup(D9,$G$3:$H$5,2,false),""),"")

or just
=IFError(F9-Vlookup(D9,$G$3:$H$5,2,false),"")


Regards.

I'm still not sure as you didn't put in the workbook expected results because in your note in cell Q9 notes you want the durations noted in 4 rows to be subtracted by start date in col O.
Are you looking for something in E9, E10, E11 and E12? If so, what results do you expect in each of those cells?

but at least for now from what I can see this might be what you need...
=if(d9="low",O9-14,if(d9="medium",o9-30,if(d9="high",o9-60,"")))
then format as date. I put that formula in cell E9

Great! AND thank you for the rep!