Saturday night, I noticed that the clock on my zip was showing the wrong time and had obviously recently started a new day as activity was nowhere near as high as it should have been by evening. Now, checking activity, I have very little activity for Saturday and nothing for Sunday. Also, shows that I'm still syncing, but no activity posting, apparently. Clock has corrected upon syncing.
The exact same thing has happened to me several times now. After months of wearing my zip and having it work flawlessly, suddenly it is not working right at all. Syncing and sometimes taking the battery out fixes the time, but I lose the steps that were already counted. I no longer feel like I can count on my zip to keep track of my steps.