Return previous Sunday Date

Hi, I'm trying to return the date of the previous Sunday based on the date selected by the user. 

I have this measure which works based on today; TODAY() - WEEKDAY(TODAY(),1)+1 

But if I swap out the today() with selectedvalue or even a variable with the selected date stored in I get a random date of 31/12/1899 so I assume it has something to do with not recognising my dates as dates? In any case everything I have tried so far hasn't worked.

What am I doing wrong, anyone? 

Thanks,

Ben