How to disable caching of a block? Although in Performance, the Block caching is disabled but I've a block created by Clock module(http://drupal.org/project/clock) when accessed as anonymous user it does not update the clock on every page reload.

I've even tried clearning browser cache but it works fine when logged in. For registered users it seems block caching is disabled.

The Clock block is there in this site: http://bit.ly/PKI2M8 - please scroll to bottom-most right block titled 'Rail Time'.

How can I disable caching of this block?
As MPD pointed out, page caching is your problem. Even with block caching disabled, the whole rendered page will be cached with page caching on, which includes any blocks no matter whether block caching is disabled or not.

As far as I know that leaves you with at least the following options:


Take the easy route, and just disable page caching.
Use JavasSript to set or update the time client side, you would
still need an initial time source though, perhaps as outlined
here.
As a much larger project you could see how authcache works for
serving it's blocks of user specific non-cached content while the
majority of the page is cached, and implement it for anonymous users
for just the time block.
Use an iframe and get the contents from somewhere it isn't cached.
There are probably more ways...