By default, the %%image-date%% will output the date and time the Instagram media was added to Instagram in the format M d, Y @ H:i, eg. Oct 05, 2017 @ 18:49.
This format can be customized with the ‘my_igp_date_format’ filter:
You can see what format options are available here https://www.php.net/manual/en/datetime.format.php