There isn't really any difference. One describes 'any' example of it and the other describes 'all' examples of it. So logically they're the same. It's like saying: Any breed of dog has four legs. All breeds of dog have four legs.

“At such a time” can refer to a specific moment in time and the specific conditions that created it. “At such times” is more general and describes a more generalized understanding of how conditions can lead to such moments