This is exactly the same problem i am having but in reverse - you are ahead of GMT so you get yesterday's date, i am behind GMT so i get tomorrow's date. My initial testing shows this only happens when you sync via the free.kindle.com email address - it doesn't happen when you sync via USB. I believe this is still a timestamping problem on the MOBI that Calibre generates, but i haven't had time to fully test it yet. See
here and
my other linked thread for more discussion.