A date field is actually stored as a timestamp, which is just an integer understood to be "seconds since some date".
In order to change how your date field displays in AGOL, go into the Configure Popups → Configure Attributes dialog in a webmap:
According to the Pro docs, the date field display is determined by your machine settings. I do wish that there were more formatting options for datetime fields in Pro.
- Josh Carlson
Kendall County GIS