The question is raised about why so many museum job postings allow very
little time for someone to submit an application, and the anticipated
"start date" for the successful applicant isn't mentioned.

Are many, if not most, of these postings simply an effort by the museums to
be able to verify that their openings were "advertised" widely in the
effort to seek non-traditional applicants, i.e., to comply with affirmative
action policies?

In so many cases, I am afraid, the museum has already pre-selected someone
for a job but either government regulation or museum policy requires that
every vacancy be advertised.  I'd be curious how many museums really do
open their best positions to all who might be qualified/interested....and
then select from that pool without prior familiarity with the applicants.