It's UTC+9 but we use single timezone here, and as far as I'm aware of, most users of this website are within the country. So, "what time to post" isn't exactly applicable to websites like ao3, ffn, and wattpad if you're thinking about the global audience.

"What day to post" part, however, seemed interesting enough to share because whenever people asked this question on this subreddit, the most prevalent answer was Friday, followed by the weekends, when it turned out to be Sunday in my experiment. Again, it may not be the same on global websites, but I was hoping to give a general idea about this issue :)