Why did the air defense alarm on 2065438 sound at 10.5. 18?

Hehe, this is an air defense alarm sound. There are text messages on the mobile phone, and the newspaper also publishes them one day in advance. The city TV station has a scrolling subtitle prompt.

Early warning: sound for 36 seconds, stop for 24 seconds, and repeat for 3 times as a cycle; Air raid alarm: sound for 6 seconds, stop for 6 seconds, and repeat 15 times as a cycle; Disaster warning: sound15s, stop10s, sound for 5s, stop10s, and repeat for three times as a cycle; Deactivate the alarm: continuous sounding for 3 minutes ~ ~ The whole sounding process needs to last for about 20 minutes (including intermittent time).