Can't find date metakey

Great plugin.

I’m looking to do the same as pixelkicks https://www.ads-software.com/support/topic/filtering-on-month-value-only-of-an-entire-date-field but can’t seem to find the date or time metakey listed in the Meta key drop down. Obviously every theme is different but I’ve tried every Meta key listed and none of them seem to be the date. Is there a way to create a meta key based on the date or month?

Also, my client wants to filter by author, is this possible?

Thanks,
Mat

https://www.ads-software.com/plugins/ajax-wp-query-search-filter/