Get Previous Month

I'm not sure what you want done.

A simple formula to get the previous month as a month name could look something like in a formula field:

let x := month(DateField) - 1;
if x = 0 then "December" else monthName(x) end

I don't know if you are basing it off a date field or just using the current month.

To calculate a previous month you also need to allow for the fact that this might move the date to the December of the previous year. Use something like

let t := today();
let d := day(t);
let m := month(t);
let y := year(t);
if m = 1 then
    m := 12;
    y := y - 1
else
    m := m - 1
end;
date(y,m,d)

Use whatever field you need to in the first line.

Regards John

Why not just use:

date(year(DateField), month(DateField)  -1, day(DateField))