Dynamic time statements?

I've recently been looking at putting conference data into a
triplestore with a SPARQL front-end . It will be desirable to ask
questions like: "Who's presenting now?", "What did they post on their
blogs in the last 24hrs?"

I'm contemplating a statement generated on the fly:

http://purl.org/stuff/datetime#Now rdf:value "2005-06-24T15:05:00Z"

refreshing in the store immediately prior to each query.

Following a couple of comments [1] when I mentioned this on my blog,
I'd be interested in hearing if and where the logic screws up with
this, and whether there aren't other (easy) approaches.

Cheers,
Danny.

[1] http://dannyayers.com/archives/2005/06/24/dynamic-time-statements/

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