Function format_unix_timestamp 

pub fn format_unix_timestamp(unix_secs: u64) -> String

Formats Unix seconds as an RFC3339 UTC timestamp (YYYY-MM-DDTHH:MM:SSZ) without pulling in a date/time crate: this is the one place in the codebase that needs calendar math, so a compact civil-from-days conversion (Howard Hinnant’s well-known civil_from_days algorithm, see http://howardhinnant.github.io/date_algorithms.html) is a fair ponytail trade against adding chrono/time as a dependency for a couple of call sites.