Ultra simple way would be to pop some javascript into a content editor webpart on the page.
There's a working sample here;
http://stackoverflow.com/questions/18536726/javascript-to-display-the-current-date-and-time
Unfortunately it's not that easy anymore. See the following link for a possible solution:
Hi NSIISLC,
How did you add the code into the Content Editor web part?
You need to edit the content editor web part, then click "Click here to add new content", then click "Edit Source" and add your code in HTML source, like:
Then click OK, and save your page.
In my testing, the result is:
Thanks,
Wendy