Is the date parameter being passed in the URL or is the user entering it? When you create the Date Time parameter in the designer you can click the change button next to the "format as" field and select a format from that list. If none of those work you can create a custom format with the format code box at the bottom.
The user is entering the date.Now I tested modify the parameter in the designer, as you explain, however does not work.
Attached a picture. Any other suggestions?
A detail, I call the report from php !