customization: replicate the value of one ACF field in another

I have a CPT that uses custom fields (created through ACF). One of them is a repeater type. Works.

Between the subfields, I have an input field called party_time. In many cases party_time Values ​​have format day.month.year, but unfortunately it also includes ranges such as 4-5.05.2020.

What I am trying to achieve is to create another dynamic field that shows values ​​(both in backend / frontend) of party_time but limited to the last 7 characters. Thanks to that I will always have something like: 06.2020 / 02.2020 etc. So basically = month format.year.

How to achieve that? I have reviewed all the official documents and I have no idea where to start. Why do I want to do it that way and NOT use only substr in the user interface? Because I want to have the ability to filter wp_query by this subfield (month.year). The cloning type field is not appropriate because it only shows the same value, without the possibility to customize it.

Thanks for any advice …