QV is very happy to extract data from Excel spreadsheets (you can use a wizard when writing the script). So this would be a good option. Create a spreasheet with a (single) date in it which your user can update. You may need a little validation in the script to make sure your user has asked for a 'sensible' date.
Just adding a note: there is also a system function to query the user to input a value during script execution:
Input( str cue [, str caption] )
This function can be used in the script only and opens an input box that prompts the user for a value during the script execution. The parameters cue and caption are used as message and caption texts, respectively. The function returns the entered value.
The input box function returns NULL if the dialog is cancelled, closed or cannot be shown.
Input('Enter value', 'Input box') as v,
Recno () as r