Agenda has a confusing usage of timezone

When a user creates an event, it's done in the user browser timezone and the time used in web interface or mail has no timezone.

create an event between 12:00 and 14:00 with browser in UTC

display the event with browser in UTC, it show time between 12:00 and 14:00

display the event with browser in fr_FR for units, it show time between 11:00 and 13:00

mail in HTML received as a guest of the event display time at 11:00 without timezone information

Vous avez été invité à un évènement

Votre évènement "Manger" a été programmé par daniel.dehennin@region-academique-bourgogne-franche-comte.fr le 13 janvier 2023 11:00 

Vous pouvez l'ajouter à votre agenda en ouvrant la pièce jointe. 

ICS event sent as attachment it between 11:00 and 13:00 UTC

BEGIN:VCALENDAR
VERSION:2.0
PRODID:-//sebbo.net//ical-generator//EN
NAME:event iCal
X-WR-CALNAME:event iCal
BEGIN:VEVENT
UID:e8a25bdb-52e6-417a-a7f4-ef6dc199bee4
SEQUENCE:0
DTSTAMP:20230112T100235Z
DTSTART:20230113T110000Z
DTEND:20230113T130000Z
SUMMARY:Manger
LOCATION:Saint-Nicolas
DESCRIPTION:Aller chercher un truc à manger et se poser dans un endroit s
 ympa
URL;VALUE=URI:/event/fzocdTtJmiioNh7NT
END:VEVENT
END:VCALENDAR