Eli Zaretskii <eliz@gnu.org> writes:

> > -;;;### (autoloads nil "cal-iso" "cal-iso.el" t)
> > +;;;### (autoloads nil "cal-iso" "cal-iso.el" (22225 49314 854982
> > +;;;;;;  791000))
>
> AFAIU, these differences are meaningless.  But I have no autoloads
> whose time stamp is 't'.

FWIW, I once saw this while calling "make" for Helm.  I investigated and
saw such a "t" timestamp in the loaddefs file.  I didn't think long
about it.  I just deleted that loaddefs file - after that, "make"
succeeded, and instead of "t" there was a "real" time stamp.

But we are lucky, I still have a Lisp backtrace from the error in my
Trash (not from building, but from reproducing with
`update-directory-autoloads'):