Post Timeline has a variety of date formats available through which you can choose the required one, the selection of the date format is usually through the shortcode builder that overrides the Post Timeline Settings.

Post Timeline renders the date of the post/event that can be changed through the Post Timeline settings as shown in the screenshot below.

The above list offers a variety of date formats, however, in case you want something different, it can be done easily via the shortcode attribute date-format=”Y”

[post-timeline date-format="m-Y"]

You can get any PHP-supported date format and add that within the double quotes, the possible options can be seen on this PHP Dates.