The Mark by Jen Nadol
Sixteen-year old Cassie Renfield has seen the mark since forver: a glow around certain people as if a candle were held behind their back.
The one time she pointed it out taught her not to do it again, so Cassie has kept quiet, considering its rare appearances odd, but insignificant. Until the day she watches a man die. Mining her memories, Cassie realizes she can see a person's imminent death. Not how or where, only when: today.
Most importantly, if you know today is someone's last, should you tell?