slow and fast formats?

fmt_fast.js

// Test Scaffold
const colors = { normal: x => process.stdout.write(x), bold: x => process.stdout.write('<b>'+x+'</b>') };
const newline = () => console.log(); // starting to be a pattern here.
const obj = { builtAt: 0 };
const fmt = new Intl.DateTimeFormat([], {
	day: "2-digit",
	month: "2-digit",
	year: "numeric",

	hour: "2-digit",
	minute: "2-digit",
	second: "2-digit"
});

// https://github.com/webpack/webpack/blob/master/lib/Stats.js#L850
const builtAtDate = new Date(obj.builtAt);
colors.normal("Built at: ");
for(const {type, value} of fmt.formatToParts(builtAtDate)) {
	if(type === 'hour' || type === 'minute' || type === 'second') {
		colors.bold(value);
	} else {
		colors.normal(value);
	}
}
newline();
// Built at: 1969-12-31 <b>16</b>:<b>00</b>:<b>00</b>

fmt_slow.js

// Test Scaffold
const colors = { normal: x => process.stdout.write(x), bold: x => process.stdout.write('<b>'+x+'</b>') };
const newline = () => console.log(); // starting to be a pattern here.
const obj = { builtAt: 0 };

// https://github.com/webpack/webpack/blob/master/lib/Stats.js#L850
const builtAtDate = new Date(obj.builtAt);
colors.normal("Built at: ");
colors.normal(
	builtAtDate.toLocaleDateString(undefined, {
		day: "2-digit",
		month: "2-digit",
		year: "numeric"
	})
);
colors.normal(" ");
colors.bold(builtAtDate.toLocaleTimeString());
newline();
// Built at: 1969-12-31 <b>16:00:00</b>

see: https://twitter.com/srl295/status/1149745866422534144