Copper accepts MM/DD/YYYY as a date format for its custom fields. Here is how to manipulate Zapier to take your date.

1. Add an action called Formatter

2. Make the action look as below (zoom in to see the image)

3. Make sure the From and To Format are both UTC

You can also put a timestamp in for NOW:

https://support.copper.com/hc/en-us/articles/115002462446-How-to-add-a-timestamp-in-Zapier