"Base delay on date field in trigger" doesn't work with custom date.
When editing "Add action to contact" and trying to use a custom date field, activity is created with the present date and not with the custom date. In this case "Take the date and forward 90 days before"
However, when I'm using either fields "Birth date" or "deceased date" I can get the exact date according to the condition I've set.
I have also checked the custom field date format in the database, and I've set the same format as for "Birth date" and "Deceased date". All of them have the "Date" format. Previously the custom field had "Datetime" format.
So it seems that the custom date field is simply ignored.