Open File With Previous Month In Name

I have a macro that saves a workbook with the month name automatically generated in the filename with the code below (apologies for the scrappy code, but I am rather new to VBA)

ActiveWorkbook.SaveAs Filename:= _
"I: EngineeringCapacity PlanningCapacity Report CONTROLS " & Format( Date, "mmmm") & ".xls" _
, FileFormat:=xlNormal, Password:="", WriteResPassword:="", _
ReadOnlyRecommended:=False, CreateBackup:=False

In another file I want to re-open this workbook. However, the problem arises because this will not happen till the next month, in order to update the new month's workbook with the old information. Currently I am 'cheating' by modifying the code with the actual month name, like this

Workbooks.Open Filename:="I:EngineeringCapacity PlanningCapacity Report CONTROLS April.xls" I have tried to use the Dateadd function (with a -1 as the add amount) but I have failed to make it work.