Can't open calendar on taskbar [Win7]

First off, I wouldn't be asking if Google had provided a solution to my problem but even after hours of reading different sites / forums, I legitimately couldn't find anything for Windows 7.

You know how there's the time and date in the right-hand corner oh the taskbar? When you click on it, the calendar opens - except mine doesn't do that anymore. It just randomly stopped two days ago. Time and date is still shown, though when I click on it, nothing happens. I restarted the PC, nothing. Reset the calender options in the "date and time" settings, didn't help either.

You really do only notice how important things are once they stop working. Does anyone know or has an idea of what could fix this? I'm seriously lost on this one.

Thank you for reading.

For Windows 7:

Check HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\SideBySide\PreferExternalManifest and make sure it is not there or set to 0.

If set to 1, it will affect the behavior of clock/calendar popup does not display.Restart your machine after you change the value.

You should run sfc /scannow in cmd as administrator to fix the corrupted system files. Create a new account, log in to see if the problem still exists.