Here I changed .PriorityDays to Param.PriorityDays and "ING" to "USDefault" for my testing as I don't have that property and business calendar ING on my system. Also I used time zone America/New_York. Please change based on your requirement.
If you see in below screenshot .pxCreateDateTime is given as June 12, 12.28 PM in America/New_York time zone. June 12 is Friday and when we add 1 to that it returns June 15 with time as 16.28 in GMT which is same as 12.28 in America/New_York.
I saw another function @DateTime.TimeDifferenceBusinessDays() which I thought might work based on description provided. I couldn't try as it was giving some error for me.