Even if zabbix proxy is online, monitored host with wrong time will force housekeeper not to remove records from proxy_history table for up to ProxyOfflineBuffer.
Steps to reproduce:
Add few hosts to monitor via proxy.
On one host disable ntp and set time to, for example, now() - 3d.
records in proxy_history table stored for 2 days.
records deleted from proxy_history by housekeeper since proxy is online and ProxyLocalBuffer=0.
There are two "clock" fields in proxy_history table:
timestamp from monitored host written to clock
timestamp from proxy itself written to write_clock
housekeeper uses clock field to filter records, so wrong time on monitored host breaks delete logic.