Date format changing when converting to text

I have a date slicer that constantly updates with new dates, but I also always need the latest date selected. 
the only way I have found to do this is to change the MAX date to text "Latest Week"

So my slicer looks like this;

this way the 'Latest week' selection is always the largest date, and I dont have to constantly republish the report with a new date as the default. 

The problem I have is when I convery the 'FW as Date' column to text, it changes the format from DD/MM to MM/DD. 

See below; 

fw as week text = 'Lettings Fact_Lettings_Summary'[FW as Date].[Date]

The original date column is correct. 

How can I stop this? Thanks!