Add "legacy" time tracking as CLOCK entries

[Richard Walker <walkerrichardj@gmail.com>]

Hi, while I've been using org-mode for some time, I'm now working
on a project where I've decided to use the time-tracking functions.
But I've got some "legacy" time on the project that I would like
to incorporate. I.e., I have in my (paper) log book a record of
(say) half an hour spent on a certain day, but I don't have a record
of the start and end time and I don't want to invent them.

I note that I can add an entry that looks like this:

    CLOCK: => 0:30

and have this contribute to totals. But I would
also like to record the date for this entry.

Checking the regular expression in org-clock-sum
and a bit of trial and error showed that
this _doesn't_ work:

    CLOCK: [2013-12-11 Wed] => 0:30

and nor does:

    CLOCK: [2013-12-11 Wed]--[2013-12-11 Wed] => 0:30

But this does seem to work:

    CLOCK: => 0:30 [2013-12-11 Wed]

I've figured this out myself on the basis of not finding anything
about it in the manual. Nevertheless, is there a "standard"
way of doing this that I missed?