Karen,
I’m not sure if this can be achieved via a popup, but I have a few dashboards that include reports for cases and activities created by month. I have two date pickers on the screen. One is hidden and is set to the current date/ time. The other is set by the user. Then in step 2 of 4 on the report object wizard I have a filter:
Field: <date field in the table of the report>
Filter: Greater than or equal to
Get value from: Value from screen
Select value: <your visible date picker>
Set the actions on the date picker to refresh the report when the date is changed.