I dug into this bug, and there are actually three distinct symptoms sharing a single cause.
The cause
The front end uses dayjs for all date calculations. A dashboard box (EDF Tempo) loaded the timezone plugin of dayjs to read the time in Paris.
The problem: dayjs.extend() globally modifies dayjs for the entire application. And the calendar library (react-big-calendar) contains this test:
// if the timezone plugin is loaded, then use the timezone aware version
const dayjs = dayjsLib.tz ? dayjsLib.tz : dayjsLib;
As soon as the plugin is present, the calendar switches all its calculations to dayjs.tz, which behaves differently:
it interprets certain dates as UTC → the time grid shifts by 2 hours
add(1, ‹ jour ›) adds 24 absolute hours, whereas the day of the winter time change lasts 25 hours → we fall back to 23:00 on the same day, hence the date displayed twice
startOf(‹ mois ›) shifts by one hour, which distorts the comparison of months → the greying inverts
Important point: this box was statically imported with all the others, so the plugin was loaded even if you never used EDF Tempo.
The fix
Instead of patching the calendar, I removed the cause: the EDF Tempo box now uses Intl.DateTimeFormat (native, without side effects) to read the time in Paris. The business behavior is identical — the full/off-peak hours remain defined in French time regardless of your time zone.
There remains a fix in the calendar, for a bug independent of this one: the day of the time change displayed 26 time slots in October and 22 in March instead of 24, and the events were mispositioned (an appointment at 14:00 was displayed at 13:26).
