Function format_http_date 

pub fn format_http_date(unix_secs: u64) -> String

Formats a Unix timestamp (seconds since 1970-01-01T00:00:00Z) as an HTTP date string, e.g. "Thu, 01 Jan 1970 00:00:00 GMT".

Implemented without any date-time library using the civil-calendar algorithm by Howard Hinnant.