One of our customers had the same problem. We worked with OpenERP support to have this fixed. It was partially fixed, but our customer decided to migrate to 7 once they saw how timesheets worked in that version.
On Windows, the web client timezone preference is ignored and the web browser timezone is used. This was part of the problem (and I've seen a warning in 7 when the two don't match).
I can dig up the patches and bug reports if you want them, but we didn't finish fixing the problem because our customer moved on.
Hope that helps!
Ray CarnesUrsa Information SystemsOpenERP Silver Partner
Our OpenERP modules can be found here