Types date fields are stored as unix timestamps, which records the number of seconds since 1 Jan 1970.
With timestamps there is no "Today", there is only a specific time, e.g. midnight 5 June 2019, which has a timestamp of 1559692800. (You can try this out for yourself at unixtimestamp.com.)
So if you want to add a filter to show posts from today you need to be comparing as a number (e.g. 1559692800) not a date, and you need a between comparison, i.e. between midnight yesterday and midnight today.
The way you would add such a filter is shown in the screenshot.
Thank you Nigel,
is it possible that every day at 23h59, the custom field'out' which is a radio is set to 1 for all those who have not validated their outing?
Toolset doesn't offer such functionality but you can use WordPress's cron jobs to trigger custom code that you write to update custom fields.
You set up the schedule with wp_schedule_event, and your code will need to run a get_posts query to find the posts which need updating and then update them using update_post_meta.
You shouldn't need to access the database directly using SQL, you should find WordPress functions available for everything you need to do, but if you want to study the database you'll see that the posts data is stored in wp_posts, and custom fields are stored in wp_postmeta.
