The spark framework was released with Flex 4.0, and it still does not have a spark-only datefield and datechooser. I found, that in most of my projects the MX Alert, datechooser & datefield were the only MX components. Tink has made a really good Alert for Spark, and Alex Harui has made a Spark datechooser, which functions almost OK, but its skin is very ugly.
So i decided to make a usable DateChooser from Alex's component - after a while i found out, that i needed to rewrite most of the source.
So with this component you'll be able to get rid of the MX framework, making the size of your application several hundred KBs smaller. And it functions on mobile too (i guess... havent tested it).
I have only implemented a few functions from the MX component (for example you cant select a range or you cant use styles) - but on the good side the code is really simple, so you will have an easy time to extend it.