ISO 8601 to local and relative datetime plus svelte ticker component

README.md

ISO 8601 to local and relative datetime (moment.js alt)

convertUTC0ToUTC8.ts

function convertUTC0ToUTC8(timestamp: number) {
    // Convert Unix timestamp (UTC+0) to milliseconds
    const dateUTC0 = new Date(timestamp * 1000);

    // Add 8 hours (UTC+8) in milliseconds
    const dateUTC8 = new Date(dateUTC0.getTime() + 8 * 60 * 60 * 1000);

    // // Format as RFC 3339 (YYYY-MM-DDTHH:mm:ss+08:00)
    // return dateUTC8.toISOString().replace("Z", "+08:00");
    return dateUTC8.toISOString();
}

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 SECOND_MS = 1000;
const MINUTE_MS = SECOND_MS * 60;
const HOUR_MS = MINUTE_MS * 60;
const DAY_MS = HOUR_MS * 24;
const WEEK_MS = DAY_MS * 7;
const MONTH_MS = DAY_MS * 30;
const YEAR_MS = DAY_MS * 365;

const fmt = {
    "year": YEAR_MS,
    "month": MONTH_MS,
    "week": WEEK_MS,
    "day": DAY_MS,
    "hour": HOUR_MS,
    "minute": MINUTE_MS,
    "second": SECOND_MS,
}

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

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

    let odiff = creation - now
    let diff
    let unit
    for (const [u, ms] of Object.entries(fmt)) {
        unit = u
        diff = Math.ceil(odiff / ms)
        if (diff) break;
    }

    return rtf.format(diff, unit);
}