Looks like you're not using the correct date format. It should work just fine (even with limit=50) if you set
MWDATEFMT='%Y%m%d%H%M%S'
At least, this worked without any problem:
site = mwclient.Site('en.wikipedia.org')
page = mwclient.page.Page(site, 'Main Page')
rev = page.revisions(start='20080902000000', end='20090902000000', dir='newer', limit=50...
2009-09-02 00:56:56 UTC in mwclient
