In a "Close" validation, I need to check that the issue linked to the current issue with link type name "linkA" has a custom field "MyDate" (type Date) that is not empty.
You can do it using a Boolean validator with math, date-time or text-string terms using the following configuration:
Text to be parsed is: count(linkedIssues("linkA")) = count(fieldValue({00012}, linkedIssues("linkA")))
Note that: