This is no longer a plan, but implemented as of Mercurial 0.9.3, but documentation is still missing.

hg log:

There is a -d/--date switch to show log entries from particular dates. Supported formats:

We'll need to scan the entire changelog as there's no guarantee that there's any date ordering.

hg update/revert:

-d/--date results in scanning the log for the first matching changeset. This is usually what people want.