If it's any consolation, this speaker of BrE has no idea. Could you give more context? Where did you see it?
1. Your question would have been better expressed as, "What does the American time expression "8/7c" mean?"
2. Please try to give your threads titles that give us some idea of what to expect when we read your question. "Meaning of '8/7c.?" would have been perfect for this.
Student or Learner