>my forms's date format is dd-mm-yyyy (set date italian dmy)
>Now, I want to show it in report as yyyy-mm-dd (in string format )
>How can I save the date format form dd-mm-yyyy into string format (yyyy-mm-dd)

I think the easiest solution would be to use

SET DATE YMD
SET CENTURY ON

right before calling this report and save the current settings into variables and restore them after the report execution.

If you want to only display particular date in this format and not all the dates, you may use TRANSFORM function to transform to the format you like.