I do your presentation props — PowerPoint slides and templates
It’s easy to change the footer only once for all slides in a presentation.
See attached screen grab for PPT 2007
And a screen grab for PowerPoint 2013. Pretty much the same thing.
In PowerPoint 2007 (quite similar in PPT 2010), follow this path to access the editing possibilities for your slide master’s footer:
Insert menu/Text section/Header & Footer button/Slide tab/
Type-in footer changes, in this case a date change and click “apply to all”.
Now that was simple.
And if it still won’t apply your changes, you may need to do as Roy does on The IT Crowd. “Have you tried turning off and on again?” I’ve found that for a few slides, the location or style is different than on the master, especially when you’ve dragged in slides from somewhere else. This is when I’ll do a reset on the footer (off and on again). Microsoft forgot to put a reset button.