How to use process calendar in a!subtractDateTime() ?

I have a requirement, which is to query data within two working days.

I use process calendar and a!subtractDateTime() to achieve this functionality.

I first defined the holiday in the process calendar, and used a!subtractDateTime() to calculate the start time of the query.

Code:

a!subtractDateTime(
startDateTime: now(),
days: 2,
useProcessCalendar: true,
processCalendarName: "MasonTestCal",
) & " ---- " & now()

The out put is:

My account is set up as follows:

Questions:

1.Why are the calculated results not different by 48 hours?

2.What are the specific rules for a!subtractDateTime() when using process calendar?