I don't know why you would enter time and then add it to "no time to report"? Is this some accounting thing where all hours need to be accounted for in the end, and you just need to split un-billable hours from billable ones? In that case I'd expect there to be an "Overhead" project to be used for that. After all, in the end someone still pays for those hours (even if it's your employer with your monthly wage).
If that's not the case, what one usually does is to put a check box in front of items that may be left out. You can deactivate that check box to indicate "no value" or "no change". Take older versions of iTunes, for example, where you could select multiple tracks, and then chose which properties to bulk-edit by checking the ones you want to change.
PS - as others have pointed out, the checkboxes are awkward and complicated UI. Usually you'd either have a text field where people can enter a date, (this is useful when people frequently use your UI, because typing one key is faster than moving a mouse and clicking for each day once you're acquainted with the workflow), or you provide a calendar where people can select days.
If there's some CSS magic you plan to do to turn the checkboxes into a calendar, be sure to permit drag-selecting multiple days & keyboard entering of a date.