Provide a sane syntax for date ranges. First, we don't want to require
both endpoints to be specified. For example it would be nice to be
able to say things like "since:2009-01-1" or "until:2009-01-1" and
-have the other enpoint be implicit. Second we'd like to support
+have the other endpoint be implicit. Second we'd like to support
relative specifications of time such as "since:'2 months ago'". To do
any of this we're probably going to need to break down an write our
own parser for the query string rather than using Xapian's QueryParser