Function format_date_time 

pub fn format_date_time(time_stamp_millis: i64) -> String

Produces the filename for a daily log file given a Unix timestamp in milliseconds.

The filename takes the form log_YYYY-M-D.bin. It is derived entirely from the provided timestamp rather than from Utc::now(), ensuring that entries processed after midnight due to channel queue lag are still written to the correct file.

If the timestamp cannot be converted to a valid DateTime, the function falls back to the Unix epoch (1970-01-01) via unwrap_or_default.