The name of the call is JournalForWorkflowUnit. The Workflow Unit is basically like the more-commonly-known "Level 1" Data Unit, i.e. the set of data identified by the combination of one member for each of the 6 main dimensions (i.e. 1 cube, 1 scenario, 1 entity, 1 parent, 1 time period, 1 consolidation member), with some further restrictions. That means you are restricted to one time member.
A possible workaround I can see is to set up the adapter to require a Parameter for the month, then execute the adapter multiple times in a Dashboard DataSet using BRApi.Dashboards.Process.GetAdoDataSetForAdapter, passing a different month each time. You can then unite the resulting DataTable objects with their .Merge method, and return the resulting object.
Another workaround is to dump everything to file, using BRApi.Journals.Data.ExportJournalsToCsv (which accepts a time filter), and then read the file back into a DataTable.