I have had this issue and could not figure out the problem. In buddypress next to a user's name it says: Active 7 hours ago...even for myself, active that second it would still say Active 7 hours, 2 minutes ago, for example. I finally went through and deactivated plugins one by one, and when I deactivated the document manager, the issue went away. It now says last active 4 minutes ago for me.
So, an issue with your plugin conflicting with the wordpress time. I do have time zone as Los Angeles, but I have also tried it with GMT and same result.
Is this a general issue with your plugin or something else? Please advise.