Plain Today Calendar application provides Notification Center Calendar Widget for your Mac (with additional features)

When you click on a day on the calendar, either - the installed OS X application calendar will be launched for that date; or - the date will be copied to the clipboard. These actions can be adjusted in Preferences.

Additional features include: - the Basic Date Calculator, which allows users to add or subtract periods of time from certain dates and includes a business-day counter; and - the Date Difference Calculator.