AUTOMATION - Epic Target Date based on Child Created Date

[Bryce Remington]

Hello,

I am trying to create an automation that will allow me to look at all linked child User Stories of an Epic and then set the "Target start" (custom field for portfolio) of the Epic based on the earliest created date of the User Stories.

Example:

Target start = Blank (Epic)

Epic Child Issues:
User Story - Created 4/19/2023
User Story - Created 4/18/2023

I want the ability to set the Target start of the Epic to = 4/18/2023 - which is the earliest create date of a child user story under it. 

I created a flow that does the following:

1. WHEN: Trigger when item is updated

2. IF: Issue Type = Epic

3. AND: Compare two values

{{#if(equals(issue.customfield_10115,null))}}TRUE{{/}}

EQUALS

TRUE

4. THEN: Edit Issue Field
{
"fields": {
"customfield_10115": "{{issue.subtasks.created.toDate("yyyy-MM-dd").min}}"
}
}

But I get an error each time that says:

Error editing issues

xxxx-8484 (Unparseable date: "" (customfield_10115))

So I figured I am setting something up incorrectly.