This plugin has not been updated by its developer since a long time ago.
It could imply partial or total lack of compatibility with the latest versions of b2evolution.
CountDown tells you years, months, days, hours, minutes, seconds between the date you set in the widget parameters and now. If you leave the "time" setting at 00:00:00 it counts in years and months and days. It counts up from a past date, or down to a future date. You get to pick the title for the widget, the text for before the count, and the text for after the count. Who could ask for anything more?