This is a
topic-of-one-thousand-threads, one of the many that keep popping up in here over and over again.
The short answer, for those that somehow managed to find this thread first, is that it is technically easy to do in any number of ways with various levels of niceness. No systems are in serious use because there is no demand. Which is funny considering how many times I've written this reply, but still true.
I guess if about 20 people all said "I have something that needs a provable trust-free timestamp
right now, and it would be worth 5 cents to me if you would finish your service so I can use it tomorrow" to me, I'd get mine ready for public consumption and announce it here.