D-30 is a Manga/Manhwa/Manhua in english language, Action series, english chapters have been translated and you can read them here. You are reading chapters on www.mgeko.cc fastest updating comic site. The Summary is

The protagonist, subjected to school violence, falls from the rooftop. After barely waking up from six months of unconsciousness, he refuses the help of his old childhood friend and chooses to transfer. He, who started martial arts by chance, is already growing enough to be ahead of the championship final and wants to show her changed appearance, but he hears the news that she was murdered by the bullies the day after he transferred. After reaching the finals in shock, he surprisingly takes a time warp on the first day of unconsciousness and tries to save her by getting a 30-day opportunity.