Blaze-Of-Glory wrote...
. . .It's not at all clear from the help for
text() that the date format expects epoch time. . . .

No?

So you'd expect the TEXT function has the smarts to render TEXT
(2,"dddd") as "Monday" in countries which consider Sunday the first
day of the week but as "Tuesday" in countries which consider Monday
the first day of the week in addition to being able to render TEXT
(2,"mmmm") as "February"?

The TEXT function only provides the same number formatting as cell
number formatting via the Format Cells dialog. While on-line help for
the TEXT function doesn't spell this out explicitly, it does imply it.

. . . I am still missing the
Monthname() formula I used to have in previous versions of excel.

There's never been a built-in MONTHNAME function in any version of
Excel. What version do you believe provided it? Or are you recalling
this from a different spreadsheet program like Quattro Pro?

The MONTHNAME function you had could have been provided by an add-in.
You could still use an add-in to provide custom functions in your
current Excel version.