How to change shards' expiry_time

I created a RP with 30d duration and I have some shards all of which has expiry_time at 00:00:00Z every day. The InfluxDB will start drop data at 08:00:00 because I am using CST time. However I would like it to do that at 00:00:00, so is there any way to change the expiry_time?