Look into the functions for this named:
That should help you out
You can use Date() formatting on that and change to 'YYYY-MM-DD' or however you want it.
gives per say:
In a data model where the highest date I chose was 26.06.2017 and I changed the formatting.
You can create addition field in your script:
date(WeekStart([Date]),'YYYY-MM-DD')&' - '&Date(WeekEnd([Date]),'YYYY-MM-DD') as [Weekname],
Then you can either use this field instead of your old week field or show it`s value in the text box with some condition like if(Getselectedcount(Week)=1,1,0).
For the textbox like:
2018-16-21 - try this expression
Hope this will help)