I have developed a page to define world time zone clocks online. At the bottom of the page, I list all 600+ time zones. Is this considered as keyword stuffing? Does this impact ranking negatively? Should I remove them? Thanks!
Is this considered as keyword stuffing?
keywords;ranking
Yes, that is horrible keyword stuffing.  Think about it from a searcher's perspective.  Let's say they search for Moscow Time and land on your site.  Initially they think This is great, lots of world clocks, let me see which one is for Moscow.  Unfortunately, you don't say which clock is for Moscow.  In fact, you appear not to even show the correct clock for Moscow.   The user is going to leave unsatisfied.  Your site will quickly fall in ranking for that keyword.  Google's algorithms or a manual review may penalize your site for the keyword stuffing. To rank for this many keywords you need to create multiple landing pages.   At the very least, each timezone should have its own page and you could list the cities in that time zone on that page.  You could even go further and create a page for a Boston Clock and a New York City Clock, even though the two are in the same time zone.