Zoho Flow creating Zoho CRM entry and data type problem

Hi all,

I'm trying to connect Calendly with Zoho CRM. From a logic perspective, I'd like the following to happen.

Client schedules a call in Calendly

Call is added to Zoho CRM record

A Task is created with a due date 14 days after the scheduled date of the call

The call is added just fine - no issues there.

When I try and create the follow up task, I'm running into issues, specifically with the due date and data types. No matter what I try, I get a data type error on the CRM side. CRM is receiving a string, not a date.

The way I'm trying to do this is I created a custom function to take the schedule date and add 14 days to it and use the new date as the due date for the new task. Here's the custom function - it outputs the correct value but CRM rejects it as a string.

date Calculate_Followup_Date(date start_date)
{
followup_date = start_date.addDay(14);
followup_date = followup_date.toDate();
return followup_date;
}

I have read and reread the documentation but don't see what I'm doing wrong. Am I missing something? Seems like this should work but I can't seem to get anything out of any custom function other than a string.