I figured it out, the old database had empty entries for the custom date for each issue. 
I simply removed the empty values and the issue was gone.
					Search found 2 matches
- 06 Mar 2021, 13:44
- Forum: General Discussion
- Topic: Date type custom field with default value
- Replies: 22
- Views: 32890
- 12 Feb 2021, 18:17
- Forum: General Discussion
- Topic: Date type custom field with default value
- Replies: 22
- Views: 32890
Re: Date type custom field with default value
I know this is an old thread, but I'm faced with a similar issue after updating to the latest version.
We have a custom date field which is somehow causing us trouble now, which worked in the past.
the date field is empty by default and is optional.
 
date(): Argument #2 ($timestamp) must be of ...
					We have a custom date field which is somehow causing us trouble now, which worked in the past.
the date field is empty by default and is optional.
date(): Argument #2 ($timestamp) must be of ...