Steps to reproduce the issue
Error
Save failed with the following error: Incorrect datetime value: '' for column 'modified' at row 1
I am getting these error when I am create a new category in Field Groups in the Joomla! 4.0.0-alpha7 while the same process is working fine at Joomla! 3.9.4
Expected result

Actual result
If I remove the readonly attribute of Modified Date and select a date from Created Date and then copy-paste it to the Modified Date.
Then it works.!
System information (as much as possible)
Joomla! 4.0.0-alpha7
Database Version 5.7.25-0ubuntu0.16.04.2
PHP Version 7.2.16-1+ubuntu16.04.1+deb.sury.org+1
Web Server Apache/2.4.18 (Ubuntu)
Additional comments

Issue confirmed, let me try to solve this. In the table fields_groups the value being passed is '' which is not the expected datetime value.