EOMONTH with slicer

Hi there,

This dax returns first day of the previous month to today correctly. FirstDay PreviousMonth = EOMONTH(TODAY(),-2)+1

How would I replace TODAY() with my datedim i.e. slicer, so that if I went back previous months, it would still do the first day of the previous month to the selected date?

I've tried to put my datedim in place of today but this isn't correct.

Thanks

Liam