I am using the date picker calendar in a form on a MacOS 10.15.5 with Adobe Acrobat Pro DC 2020.009.20067. The customs format is dddd mmmm dd yyyy. When a date is picked the initial day is incorrect, however, when the date is selected the correct day is shown.
Sunday -> Monday
Monday -> Tuesday
Tuesday -> Wednesday
Wednesday -> Thursday
Thursday -> Friday
Friday -> Saturday
Saturday -> Jan
The pictures above show the wrong day being displayed during the picking process and the correct day being displayed after the date is selected. The problem is further compounded with console.println(this.getField("Date_af_date").value); displaying Jan June 27 2020 (the incorrect day).
That is weird. I am not getting that problem on my end.
I did noticed that at the time that you posted this question you were missing a few updates in your Acrobat version.
So here's a few ideas:
Is the field empty before you select a date?
I had a default value in the date field. I removed the default date and I still have the problem. The work around I am currently using is to type the date in the field and not pick it from the drop down calendar.
Use this guidance if repairing the installation doesn't work(or is not included in Acrobat for macOS):
Additional troubleshooting tips here: