1
00:00:00,000 --> 00:00:05,000
单独的数字，缺少含义。

2
00:00:05,000 --> 00:00:12,000
数字、单位和动作一起出现。

3
00:00:12,000 --> 00:00:20,000
按实际阅读调整，不把3秒当通用规则。