Good morning, John
Thank you very much for your answer, however it seems that my problem is more complicated than it seems…
Because I have to change the first “date” of a line of my repeater only, it is not a single date field.
I tried to modify my code as well to test:
<?php
# Affichage du Mois / Année de la période (1er Element / Date de départ)
if( have_rows('dates') ):
# Recuperation du champs "Dates"
$firstDates = get_field('dates');
# On stop sur le premier item de la période
$first_depart = $firstDates[0]['date_depart'];
// format date
$date1 = date('l d F Y', strtotime($first_depart));
// OR
$date2 = date('F Y', strtotime($first_depart));
echo($date1);
echo($date2);
endif;
?>
Here are some screenshots of my configuration of my dates fields:


I get the date of January 1, 1970
I think there is a problem beyond me.
And the result of the displayed code:

Thank you again for your help !
Bertrand
*************
Edit – Capture (IMG) are not showing…
My Repeateur Field
https://ibb.co/kGaRZw
Date Sub_Field
https://ibb.co/hPOkSG
Display on Website
https://ibb.co/g3Dr1b