秒的换算：ms(毫秒),μs(微秒),ns(纳秒),ps(皮秒)

1000 毫秒ms = 1秒

1,000,000 微秒μs = 1秒 

1,000,000,000 纳秒ns = 1秒 

1,000,000,000,000 皮秒ps = 1秒

reference: http://my.oschina.net/swingcoder/blog/1564069