what would be the impact on SEO of changing the text of the <h1> dynamically on the server side each time the web page loads?

I'm not talking about changing the whole text, just part of it, for example if the header contains some fixed text (with keywords of course), and also contains the current date or time/the current number of logged on users/the count of items current in stock/whatever.

how would that affect my ranking? is it bad? doesn't make a difference?

Thanks
Steve