To test your code, I recreated what I think you have.
I added a drop down list containing 'Today', 'Week To Date' and 'This Week'.
I added a button with this code in the 'On Select'.
UpdateContext({MyDate:
Switch(Dropdown1.Selected.Value,
"Today",DateValue("12/01/2016"),
"Week to Date",DateAdd(Today(),-Weekday(Today(),StartOfWeek.Sunday)+2,TimeUnit.Days),
"This Week",DateAdd(Today(),-Weekday(Today(), StartOfWeek.Sunday)+1,TimeUnit.Days)
)});
Collect(FromDataCollection,
{
Field:"Test Date",
Operator: TextInputCanvas1.Value,
Value: Text(MyDate),
Type:"Date"
})
Does this solve your problem?