You can't use Itsycal to add new entries or change existing appointments, but there is a button that opens the OS X Calendar app, where you can adjust your schedule. Your appointments are marked with dots and you can choose to list up to 7 days' worth of appointments below the calendar. Click on the hamburger button on the right edge to open settings, where you can link Itsycal to OS X's Calendar app (nee iCal). Click on the button to show the calendar. It installs a button in the menu bar that displays the current date. I have three to recommend, two of which are free and a paid app that does more.
Thankfully, there are a few apps that can add a calendar to your Mac's menu bar. The closest thing you get from OS X is a calendar widget you can add to the Dashboard, but it can't connect to a calendar app to show your appointments. With Windows, you can take a quick peek at a calendar by clicking on the time/date in the system tray. Unlike Windows, OS X does not provide an easily accessed calendar on the desktop.
