CM01 - Factory Calendar Day in format of 6873 instead of the date format ?

Dear seniors,

We want to display in CM01 the working days based on the factory calendar.

When I make the setting in general settings to use the Factory Calendar as a period, I get the Factory Calendar Day in another format ... 6873, 6874, etc, instead of a real date

Please advise on what is the cause of this and how it can be solved

Thank you,

Alex