Way too late for your presentation, but for future occasions you might try this?
(2 files attached)

timer_01.pptx
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- clock icon + number, counting down per slide
- slides advance automatically >> for this test I used 1.5 sec, but in reality you would use 60 sec per slide
- File >> Export >> Create a video >> timer_01.mpg (cannot attach file here)


timer_02.pptx
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- Insert Video >> timer_01.mpg
- Select video >> Playback >> Automatically
- in Animation Pane: double click Play animation >> Timing >> Stop playing >> After: 999 slides

Attached Files

	timer_01.pptx (98.8 KB, 1 views)
	timer_02.pptx (476.1 KB, 1 views)