To delete an Important Date, click on the Important Dates link on the left hand menu bar. Next, locate the date you want to delete on the table and click the corresponding pencil icon in the far right column. (Note that Upcoming Dates are listed under one tab, All Dates are listed under another.)
If you originally added the date manually, then you can delete it by clicking the Delete button at the bottom of the page. Keep in mind that this only applies to Important Dates that you created directly.
If the date was created automatically when you entered a round, such as a note maturity, warrants expiration, or options void date, you will need to edit the round itself. The same goes for dates automatically created from transactions. In this case, click the name of the round or transaction, delete the date entry and save your record.