Does AR_InvoiceHistoryHeader contain records going back 2 years?

Copy / paste your report selection criteria.  I usually write a "StartDate" formula and put the date calculation logic there, so I can drag the formula into the report for troubleshooting.

Hey, 

yes there are records in there going back 3 years.

Here is my select criteria:

{AR_InvoiceHistoryHeader.WarehouseCode} = "000" and
{AR_InvoiceHistoryHeader.CustomerNo} in ["A440", "C601", "C601PR", "C601RC", "H013", "O004", "T002", "W001C"] and
{CI_Item.ProductType} = "F" and
datediff("m",{AR_InvoiceHistoryHeader.InvoiceDate},{?PromptFromDAte}) in 0 to 23 and
UpperCase({CI_Item.ItemCode}) in UpperCase({?FROM ITEM}) to UpperCase({?TO ITEM})

Here is me building my bucket for that previous year "@Month6M-1"

if month({?PromptFromDAte})-5 = month({AR_InvoiceHistoryHeader.InvoiceDate}) and year({?PromptFromDAte})-1 = (year({AR_InvoiceHistoryHeader.InvoiceDate})) and {AR_InvoiceHistoryHeader.CustomerNo} = "A440" then "AMON6-1"

Here is my adding to my bucket "6MQtyAMON-1"

IF {@Month6M-1} = "AMON6-1" then {AR_InvoiceHistoryDetail.QuantityShipped}

then i just sum it up and put it on report.   

This gets you the start date of the chosen month, two years back.  Simple.

date(year({?PromptFromDAte})-2,month({?PromptFromDAte}),1)

ok.  is there a formula or something to make the year -1 or -2.  because when it spans years it will need to check to see if it is 2018 or 2017 to go -1 or -2 years back.  if i run it for dec 2017 it compares 2017 to 2016 with no issues and it populates all buckets fine as every one is -1.  but once it starts spanning 2 different years is where my buckets get thrown off.

You don't understand... never calculate each piece of the date individually because that makes things overly complicated.  That result is the start date of the report, two years back.  It doesn't matter whether your date falls in the middle of the year or not.  If your entered date was in Dec. 2017 then that formula result would be Dec. 1, 2015.  If you'd actually want to start on Jan. 1 2016, do this:

DateAdd ("m", 1, date(year({?PromptFromDAte})-2,month({?PromptFromDAte}),1))

Then pull invoices dated >= this value.