You can edit the SharePoint discussion list and add a calculated column that only shows the date:
1) Go to your discussion list Settings / Discussion Board Settings
2) Click "Create Column"
3) Enter Column Name: CreatedDateOnly
4) Select Type: Calculated (calculations based on other columns)
5) Enter the following Formula:
6) Remove "Created" from Discussion View WebPart and add "CreatedDateOnly"
The column, CreatedDateOnly, will display only the Date and not the time.