I would like to propose a .Internal() POSIXct2Date() function, similar
to do_POSIXlt2D() in src/main/datetime.c. I created a working
prototype, which is now included in Dirk Eddelbuettel's RApiDatetime
This prototype is a combination of asPOSIXlt and POSIXlt2D from R
Core. The functions are essentially concatenated, with the extraneous
parts removed. Directly converting from POSIXct to Date uses less
memory and is roughly twice as fast.