Lunar calendar is incorrect

I have a contact whose birthday falls on 2.Apr.2000. It shows correctly on the calendar app. No issue with this.

When I edit the contact and include the lunar calendar (birthday), the system intelligently calculate it accurately that matches the lunar calendar which is 庚辰年 二月二十八日.

But when I look into the calendar app, the day in one day ahead of the actual date, 二月二十七日 (see attached diagram)

Interestingly, I am surprise to see that all lunar calendar (birthday) will fall one day before the actual day. Is this intended behaviour or there is something I don't understand.

Oh can I steal this thread to ask a small question on how to choose the communities when I submit a question like this? I always choose iMac > using your iMac, is this correct?