Function pack_interval 

pub fn pack_interval(start: i64, end: i64) -> u64

Pack a valid-time interval [start, end] (whole seconds) into one u64: start in the high 32 bits, end in the low 32 bits. Values are truncated to i32 (≈ ±68 years around the epoch — honest demo range; a wider encoding is a file-format-v2 concern, STELLAR §C).