I decided for a workaround here, as I couldn't find a way to stop excel trying to convert datestrings to dates.

I have a client setup table in my app, where for every client it's forecast file structure is described. I added a column into this table with value TRUE when months and days are switched when the client file is opened from VBA, and FALSE or empty otherwise. When the dates are read in from source file, the last step is to check, is the check value for this client TRUE and the day number less than 13 - when yes, then the reverse switching is made.