ISO 8601 to local and relative datetime plus svelte ticker component

Ticker.svelte

<script lang="ts">
    import { onMount } from "svelte";
    import { toAutoLocalDate, toRelativeTime } from "./time";

    export let start: string;
    let date: string;

    onMount(() => {
        let interval: number = setInterval(() => {
            date = toRelativeTime(start);
        }, 1000);
        return () => clearInterval(interval);
    });
</script>

<time datetime={start} title={toAutoLocalDate(start)}>
    {date}
</time>

toAutoLocalDate.ts

function toAutoLocalDate(date: string): string {
    return Intl.DateTimeFormat(Intl.DateTimeFormat().resolvedOptions().locale).format(new Date(date));
}

toRelativeTime.ts

const lang = Intl.DateTimeFormat().resolvedOptions().locale
const MINUTE_MILLISECONDS = 1000 * 60;
const HOUR_MILLISECONDS = MINUTE_MILLISECONDS * 60;
const DAY_MILLISECONDS = HOUR_MILLISECONDS * 24;

function toRelativeTime(date: string): string {
    const rtf = new Intl.RelativeTimeFormat(lang, { numeric: "auto" });

    const creation = new Date(date).getTime();
    const now = new Date().getTime();

    let daysDifference = Math.round((creation - now) / DAY_MILLISECONDS);
    let unit = "day";
    if (!daysDifference) {
        daysDifference = Math.round((creation - now) / HOUR_MILLISECONDS);
        unit = "hour";
    }
    if (!daysDifference) {
        daysDifference = Math.round((creation - now) / MINUTE_MILLISECONDS);
        unit = "minute";
    }
    //@ts-ignore
    return rtf.format(daysDifference, unit); // ts doesn't know this unit is a Intl.RelativeTimeFormatUnit
}