Function format_datetime 

pub fn format_datetime(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.

Examples

let dt = fits_header::parse_datetime("2026-07-11T22:15:03.5").unwrap();
assert_eq!(fits_header::format_datetime(&dt), "2026-07-11T22:15:03.5");