Did you also check the timestamp? Because each date also consist of a time part and if they for some reason are different you will find no result. If you are running local you could check your postgres database to check the value.
Regards,
Ronald
[EDIT]
My advise is that you create two date objects, one the start of the day and one the end of the day. See the documentation about creating dates. Then you can use those to do an xpath compare >= or <=.