I too am having the same problem with the standard WP calendar not working. When I create a new tab and bring up the blog the calendar appears correct. Once I move off the current month for the first time, all the dates are incorrect, even when I return to the current month.
I tried the suggestions above - auto upgrade via tools, deactivate and reactivate plugins, change first day of the week and back.
I looked at the link Frumph provided to ticket 11414 and can verify the calendar works properly when I change the timezone to Europe - London but the problem returns when I return my setting to America - Edmonton.
I am unclear what the 'latest "bleeding edge nightly"' that falterago applied (last comment in ticket) to fix his problem two weeks ago. Would not doing an automatic upgrade include this fix?
Please advise what fix is supposed to be added and where. Newbie working my way through learning the code.