When you say "13 hours green" do you mean 13 hours after the arbitrary time set by the tracker you're using? or 13 hours after the well known 48h cooldown? (making it 61 hours)? If the latter I'd suggest not using those trackers as an indication for window and calculate the time yourself based on the cooldown info provided by Vandesaan, none of the trackers have been updated with the real individual cooldown of each S rank.
I was able to spawn Nandi 48 hours and 30 mins after the last kill (that is 30 mins after window opened), and that's because I went afk, I was going to try to spawn it right as the 48h timer was up.