I'm not sure what data you've been tracking, but until 2 months ago the duration of the day cycles was a variable length of time, and using a static day length like that would have resulted in a drifting countdown timer.

Every cycle was about 1 second shorter than the previous one until it was 60 seconds off, at which point it corrected itself to be ~150 minutes long again. These resets are highlighted in red on the table.

On 11/29 they seem to have fixed this to be a more stable length of time (~8998.874 seconds), however there have also been several cases where the clock has been reset early; on 1/15 one of the day cycles is only 133 minutes long coinciding with a hotfix that day, which would have permanently thrown your example code off by 17 minutes.