If you beed just a number. you could try the following: calculate the "new visitors" based on "unique visitors since xx month" minus "unique visitors since xx month but not the month in question".
example: if you define "new visitor as someone who hasn't been on a single site since 12 month", you would calculate the new visitors for last month with
a) UV in the last 13 month (12 + 1 month)
b) UV in the ladt 13 month but not last month
the difference is the amount of UV in the desired month
remark: you can get the numbers by following this steps:
1) create the rolling date ranges
2) create segments for each of the date ranges
3 create the calculate metric using the segments
side note: this solution only gives you a number, not really a segment to work with. I haven't found an easy solution to create a similar segment just using the default analytic dimensions...