Mutiple entry and duration calculation

I carry Australian passport and applied for double entry to China visa with 90 day duration. I arrived in Beijing for the first time on the 6th of December 2011. I then travel\ed to Hongkong and got back to China on the 6th of January 2012. What's the last day that I can stay in China? Is it the 6th of March (calculated based on the first entry date) or 6th of April (calculated based on the second entry date)? Thank you very much!