date boosting and dismax

[Shawn Heisey]

 I've started a couple of previous threads on this topic, but I did not 
have a good date field in my index to use at the time.  I now have a 
schema with the document's post_date in tdate format, so I would like to 
actually do some implementation.  Right now, we are not doing relevancy 
ranking at all - we sort by descending post_date.  We have been working 
on our application code so we can switch to dismax and use relevancy, 
but it's still important to have a small bias towards newer content.

The idea is nothing this list hasn't heard before - to give newer 
documents a slight relevancy boost.  An important sub-goal is to ensure 
that the adjustment doesn't render Solr's caches useless.  I'm thinking 
that this means that at a minimum, I need to round dates to a resolution 
of 1 day, but if it's doable, 1 week might be even better.  I do like 
the idea of having different boosts for different time ranges.

Can anyone give me a starting point on how to do this?  I will need 
actual URL examples and dismax configuration snippets.

Thanks,
Shawn