You will need to create a formula field to do this. I've done this using formula functions only available from Omniscope 2.5. You can also achieve the same effect using functions from Omniscope 2.4, if needed, but it is more complicated.

I've given two examples. Each uses the following sub-expression to combine months+years as a large number of months since 0AD, then subtracts one such value from another:

DATEUNIT([date], "month") + 12 * DATEUNIT([date], "year")

You end up with a number 0 to mean "current/latest month", -1 to mean "previous month" and -13 to mean "previous month in previous year". You can then filter on this value to ensure only certain months are showing. Should new data appear in the file, the formulas would re-evaluate.


First example: months from now

(DATEUNIT([Date Field], "month") + 12*DATEUNIT([Date Field], "year"))
-
(DATEUNIT(TODAY, "month") + 12*DATEUNIT(TODAY, "year"))


Second example: months from [latest month in data]

(DATEUNIT([Date Field], "month") + 12*DATEUNIT([Date Field], "year"))
-
(DATEUNIT(SUBSET_MAX([Date Field]), "month") + 12*DATEUNIT(SUBSET_MAX([Date Field]), "year"))