should files include a last-modified?

[hannesm]

I'm not good with time (and use monotonic counters instead). At the moment, each root and targets file include a created timestamp, which is never mutated. Though, each signature in these files as well contains a timestamp, so it shouldn't be an issue.

we cannot just embed a last-modified in files which require multiple signatures: updating the field would invalidate all signatures!

[cfcs]

What would the purpose be?