datefmt: microoptimize formatting

add logic to skip formatting DateTime<UTC> to UTC format using `with_timezone()` when the desired format is already UTC

src/cmd/datefmt.rs

@@ -309,6 +309,8 @@ pub fn run(argv: &[&str]) -> CliResult<()> {
  flag_formatstr = "%Y-%m-%dT%H:%M:%SZ".to_string();
  }
 
+ let is_output_utc = output_tz == chrono_tz::UTC;
+
  // set RAYON_NUM_THREADS
  util::njobs(args.flag_jobs);
 
@@ -358,9 +360,14 @@ pub fn run(argv: &[&str]) -> CliResult<()> {
  parse_with_preference_and_timezone(&cell, prefer_dmy, &input_tz)
  };
  if let Ok(format_date) = parsed_date {
- format_date_with_tz = format_date.with_timezone(&output_tz);
- formatted_date =
- format_date_with_tz.format(&flag_formatstr).to_string();
+ // don't need to call with_timezone() if output_tz is UTC
+ // as format_date is already in UTC
+ formatted_date = if is_output_utc {
+ format_date.format(&flag_formatstr).to_string()
+ } else {
+ format_date_with_tz = format_date.with_timezone(&output_tz);
+ format_date_with_tz.format(&flag_formatstr).to_string()
+ };
  if !keep_zero_time && formatted_date.ends_with("T00:00:00+00:00") {
  formatted_date[..10].clone_into(&mut cell);
  } else {