The date widget mimics an app icon but places today's date on it, to overcome this shortcoming in Android. 

Some phone manufacturers show the date on their own calendar app, but there is no general mechanism for this in Android, there is one mechanism by Nova Launcher (and adopted by some other 3rd party launchers) which is supported in aCalendar 2.

Additionally to setting the shape (to adjust to your homescreen setting for devices with Android 8+), you can configure the text below the icon to show various calendar infos, like day of week, month, year, calendar week etc.

Additionally, you can show the date in alternative calendar systems.