Now that we have date fields that internally store milliseconds (and
can currently be used in function queries) we have the basis for a
good replacement for using things like ord(date)... which is now a bad
idea since it causes the FieldCache to be instantiated at the highest
level reader... doubling the usage if it's also used for faceting or
sorting.

One issue though is that our float functions don't have enough
precision to deal with dates that well.
System.currentTimeMillis() currently contains 13 digits.  A float can
capture 7.x digits of precision.
This means that our 10^-3 seconds precision on the raw date field is
only accurate to 10^3 seconds (~15minutes) when converted to a float.

We could either:
- change function queries to use doubles internally - probably a good
idea for the future in general - seems like geo might need more
precision too.
- come up with a new date scale function that uses doubles internally?

-Yonik
http://www.lucidimagination.com

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