That was a pretty dangerous skill… any skill that could manipulate time in any form was supposed to be that insane, actually. In any case, with that one, Icarus could probably make people grow older faster. While there were probably more efficient ways to kill someone with mana, it still was pretty scary. Billy made a mental note not to let him touch him under any circumstances, even if the people of that world could live for longer. After all, Anna was sixty, and she never had any white hair.
Regardless, it seemed that those who had more tendency to work with others ended up getting powers that helped them during and after battles. Lucyna could steal anything and use those valuable things to enhance herself, and she also could cause damage with her powers. Icarus more or less had the same traits. As for Billy, his powers could only be useful in battle if he had a shit ton of mana.