Re: 28 days without generation, i have 4200khash/s

Search debug.log for "proof-of-work found".  If you find any, then check for any errors right after that.

How big of a margin on the time is allowed for things to work right.
The margin is 2 hours.

This should be solved in SVN rev 141 and the next release (0.3.11+).  It'll pop up a message box alerting you if your clock is off by more than an hour.