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!
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.
Yes that most certainly is keyword stuffing, otherwise it's really, really poor UX/writing that does an incredibly good job of emulating keyword stuffing.
First off, the IANA tz database only keeps track of about 418 distinct timezones. Many of the "timezones" you list are just different ways to identify the same zone. Many of them aren't even timezones but time offsets. And in many places the exact same timezone labels or offsets are repeated. It is literally a list of keywords just dumped on the page for no reason other than to have a bunch of keywords on the page.
There is no good reason to present a list of timezones like that. It's not user-friendly, and it does not provide a useful description of your page. Is there any reason to include Riyadh 7 times in the list? Or to represent GMT 9 times (11 times if you count the 2 random "Universal"s that appear in the list) Is there any reason at all to present an unpunctuated, undelimited list of 600 items? Do you really expect anyone to read that or to be able to find the timezone they're looking for in this wall of garbled text?
If you ever have to ask "Is this keyword stuffing? Should I remove it?" then the answer to both those questions is most certainly "yes".
Keyword stuffing is when you repeat the same word again and again.
In your page you have following
" GMT GMT GMT+0 GMT+1 GMT+10 GMT+11 GMT+12 GMT+2 GMT+3 GMT+4 GMT+5 GMT+6 GMT+7 GMT+8 GMT+9 GMT-0 GMT-1 GMT-10 GMT-11 GMT-12 GMT-13 GMT-14 GMT-2 GMT-3 GMT-4 GMT-5 GMT-6 GMT-7 GMT-8 GMT-9 GMT0 GMT0"
It look like Keyword stuffing but Google must be smart enough to detect that these are necessary information and each term represent a different meaning.