if you go with the query approach, you'll probably want to use the 
definition of "day is 0600-1800" and "night is 1800-0600".  i'm sure there 
are some sql gurus out there who could come up with a query that would do 
this?

if you go with the incremental approach, you could use sunrise/sunset or 
something more sophisticated than 0600/1800 as the cutoffs.  probably not a 
big deal for those closer to the equator, but more noticeable for those 
toward the poles.

as for implementation of the incremental approach, one way to do it would 
be to write a service that runs twice each day - sunrise and sunset (or 
0600 and 1800) and saves the previous day/night min/max to a separate 
database.  then write a search list extension that provides templates with 
access to that database.

you could write a separate, standalone tool to backfill - it would crawl 
through your existing data and bring the separate database up to date.

m

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