Foreigner living in Beijing and just got this message:

9月15日10时至10时23分，本市将在五环路外试鸣防空警报。请市民听到警报后继续保持正常工作生活秩序，关注国防人防。

Translation:

Between 1000 hours and 1023 hours on 15 September, the city will test air-raid sirens at the Fifth Ring Road. Members of the public are requested to continue to maintain normal working and living order after hearing the warning and pay attention to the Civil Air Defense Force.