I am using WP Gridbuilder for sorting and facet filters. I have stored the DateTime field as a timestamp in a custom database. If I use the sorting filter using the timestamp value. It doesn't recognize it. I have contacted them. They replied that the custom field is not associated with the post.
However, the data is associated with posts. I have checked the same data stored in the post_meta table where the sorting option is working. If I store the value in a custom table, it won't function.
The data in the custom table is associated with the post/term/user via the column ID as you can see on the documentation https://docs.metabox.io/extensions/mb-custom-table/#notes
But GridBuilder or FacetWP might not support getting data from the custom table, you can send some screenshots of your custom table (in the database) to their support and ask for help showing data.