Using Views to create a 'Tip-of-the-day' block

I'm using views to create a 'tip-of-the-day' block.
I've created a new content type for my tips, and have defined the views block to filter by this node type, select 1 node per block, sorted by 'random', and display these in a list. (Even though my list only ever has one item in it, this is the only way I could seem to choose which fields to include in the block, since I don't want to select the full node).

This works pretty well, but selects a random node on each page refresh (I assume that the seed for the random number generator is the current timestamp?). Any ideas how I could modify this to select a random node each day only?

Any help would be much appreciated. Thanks.