Hey folks - after some back and forth with support I was eventually able to solve this. There are two main ways to accomplish my own goal of “set a date X days into the future and copy it to a date picker field”.
1
The first, building off the workflow above, is to output the code-generated date as a UNIX timestamp (using miliseconds specifically - UNIX standard is seconds, but HubSpot uses miliseconds). Easiest way to do this with JS is
Math.floor(dateVariableName.getTime())
Output that value as a number variable, then use the Format Data action to coerce it to a date by using “plus_time” and adding 0 days (or any other measurement of time). As HubSpot stores its dates as UNIX timestamps anyway, it will correctly read the date, and you can use the output as a date value for a date picker.
2
If you don’t care about using a Custom Code Action to generate the date in the first place (i.e. you only need a simple date add/minus like here) you can achieve the same outcome without Custom Code.
Set a date field to the date of step using the Set Property action, take that date field and Format data to or remove the number of days, and then push that to the final date field you’re looking to set. This is by far the simpler method if you don’t have complex calculations needed to generate your date.
