Incorrect dates passed to daygridMonth header

Reduced Test Case

REDUCED TEST CASE.

Bug Description

Describe how to recreate the bug.

Use props dayHeaderFormat or dayHeaderContent to change header display on daygridMonth. In both cases the date passed to those functions is incorrect - points to Jan 1970.

What do you expect to happen?

Correct date should be passed to formatting/rendering functions just like in the other views.

What happens instead?

Incorrect date is passed - it’s always Jan 1970.

[pdcmoreira]

When FullCalendar wants to pass just dates (ignoring time) or, like in this case, just a bit of information from a date, it passes a Date object with the hours set to 00. The problem is that Date always considers the local timezone, so in places with negative offset, it becomes the previous day.

Example when working with a UTC-3 timezone:

The solution is, parse it as UTC with moment:

dayHeaderContent({ date }, h) {
  return h('div', {}, moment.utc(date).format('ddd'))
}

[Dominik-Kohler]

I would also expect it to behave like v4: Incorrect(v5):

Correct(v4):

As ‘dayHeaderFormat’ currently no longer works properly.