Current day display on day later in the CP settings, timezone is set to UTC +1 Europe/Zurich
when I try this code:

{% set currDay = now|date() %}
    {{ currDay|date('l d F Y') }}


it outputs "Mercredi, 16 janvier 2019", but we're still Tuesday 15th

Craft 3.0.37
Try

{% set currDay = now|date('M j, Y H:i:s') %}
{{ currDay }}


Applying the |date() filter to the now object when you set the variable currDay makes currDay a string. When you then try try to reapply |date() a second time, something funny must be happening..?