Rookie: Question about 24-hour cycle countdown_html/css_WEB-ITnose

This code has always been very useful. I found it yesterday when I needed it, but the hours, minutes and seconds showed NAN NAN NAN and I didn’t touch anything. I tried disconnecting my computer and adjusting the date forward by 1 month. Then I refreshed the web page and it displayed normally. I don’t understand the code, so I can only ask for help. I hope someone can help me revise it while I'm busy. It's best to mark where the problems occur so that I, a novice, can learn from it.

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Post code. . .

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