Impress: eliminating the date and time in the footer

I have some slides that had generated a <date/time> in the lower left corner, as well as defaulting centered text ‘footer’?
Some of my slides do not have this. How do I remove this <date/time> and footer nonsense from my footer on all the slides?
I have tried everything and nothing is working: I cannot even get it removed from only one slide at a time, yet alone doing it while the entire presentation is chosen (control/A) and going into >insert > header/footer…and choosing there.
Additionally, I unclicked header and my header disappeared from all slides and when I click it again it is not repopulating.
Thank you for your help!

It’s not nonsense, you just don’t know what it is. <date/time>, <footer> and <number> are only place holders that indicate where the actual data would go if filled, otherwise they should not be visible in the actual slide show (unless you somehow managed to override that), check with F5. If you see that on multiple slides it may indicate you are in the master slide view instead of the normal slide view and were adding new master slides instead of regular slides. Header/footer usually are set on the master slide(s) from which regular slides inherit, so use View → Master Slide and then Insert → Header and Footer… to edit or deactivate, or Slide → Master Elements to de-/activate for all slides. If individual slides have it and others not then they may derive from different master slides or may have header/footer individually set, in which case you can multi-select the slides and use Insert → Header and Footer… with the Apply to All button.

See also
Header and Footer
Adding a Header or a Footer to All Slides
and English documentation | LibreOffice Documentation - LibreOffice User Guides the Impress Guide to get acquainted with the concepts.

I don’t know what you mean with “I unclicked header” or which header would have disappeared.

Thank you for your help. To clarify, I do have the placeholder for date/time and footer showing on some slides, but not others, so I had been using different master slides without knowing it (choosing ‘control M’ to populate a new slide, then R clicking to choose the layout for that particular slide.
That said, I did choose the entire presentation, went to Insert-> header and footer-> and hit the Apply to all button and nothing happens anywhere (making sure that the date and time or footer were not clicked. (I do want the slide number).
With one slide, I did the same as above, chose slide number (just to see) and a second slide number appeared next to the one already there.
I did go to view normal and still no change: the same slides are displaying the placeholder for date/time and footer.
Thank you.