Hello Mark:
Thank you for posting this question. Please notice that slide master's placeholders of date type, slide number, and footer cannot be updated programmatically. If you want to update such information programmatically, please remove these placeholders from the slide master and add the corresponding text placeholders in the layouts. These text placeholders in the layout can then be replaced programmatically by using the "replace" function.
For the example of using “replace” function, please refer to the link below:
I hope this helps.
Thank you again!
