How do you prove when the work happened?

Effort has no when. A public sequence supplies one.

The last page stopped at the difficulty. The making keeps its region resident, and a public interruption cuts the stretch. Nothing in the finished file says how long that stretch was. A producer can hand over a result and say it took a day. Nothing in the bytes distinguishes a day from a decade. Why memory, and why a clock?

Someone could do the work slowly over a year in private, or run a thousand copies in parallel in a warehouse, or do it all last Tuesday and release the results one a week for the next two decades. From outside, none of that is distinguishable from the honest version. What is missing is not effort. It is a when that anyone can check.

Your own clock proves nothing, because you set it. The obvious repair is to appoint a timekeeper: a service that stamps things and vouches for the stamp. That works, and it is an office. It has to keep existing, keep being honest, and keep being trusted. Issued, listed, or made?

There is an older trick that avoids the office. To prove a photograph was not taken before a certain day, put that day's newspaper in the frame. Nobody printed the paper for you. You have bound your thing to a public event you had no hand in staging, and anyone who remembers the day can check.

Bitcoin is that trick built deliberately. A new block arrives on a schedule nobody sets. Everyone sees the same sequence in the same order. That sequence is a public interruption, and it is not a payment. Headers are small enough to hold. Reading that sequence into the work fixes the early end, and cuts the stretch along the way. It does not issue the object. The work supplies the what. The sequence supplies the when. Neither substitutes for the other.

The two limits are different. More objects need more machines. Public time cannot be bought. Someone with unlimited funds can buy their way along the first all day and make no progress on the second. The next block arrives when it arrives, at the same rate for a warehouse as for one person on a laptop.

Whether that fact is enough is not this page. Why is the check always cheaper than the making? Other networks can perform a public-tick job. None operates at Bitcoin's present scale as a public order. No second candidate is named here.