Function format_date_obs 

pub fn format_date_obs(dt: PrimitiveDateTime) -> String

Format a date/time back to the FITS civil form (YYYY-MM-DDThh:mm:ss[.fff]), dropping a zero sub-second part. Inverse of parse_date_obs.

use skymath::format_date_obs;
use time::macros::datetime;

assert_eq!(format_date_obs(datetime!(2026-07-11 22:15:00)), "2026-07-11T22:15:00");