Thanks for getting in touch.
Please try this:
DateFormat df = DateFormat.getDateTimeInstance(DateFormat.MEDIUM, DateFormat.MEDIUM, Locale.PRC);
This worked for me and correctly showed the time as well as the date.
xAxis.getLabelFormat() returns a user supplied label formatter. If you do not set this value in advance using setLabelFormat(), calling the method getLabelFormat() will return null.
I hope that this helps you, please get back in touch if you have any further questions.