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    This package exports an isomorphic function that works like setInterval, but aligns itself to the start of each day. You might use this client-side to update a UI that shows new information each day. You might use this server-side, from within a websocket handler, to push new information to the client when the day changes.


    For use from both the browser and Node:

    npm install day-start-interval

    For use in the browser, you will also need to install moment-timezone as described here, and configure your bundler to resolve that dependency. If you wish to load moment-timezone from a CDN like JSDelivr, for instance, and are using Rollup, you might do:

    <!-- in your HTML -->
    <script src="//,npm/moment-timezone@0.5.26/builds/moment-timezone-with-data.min.js"></script>
    // Part of your Rollup configuration
      external: ['moment-timezone'],
      globals: {
        'moment-timezone': 'moment'

    You may also need to configure your bundler to upconvert this module from CJS to ESM e.g. using rollup-plugin-commonjs (exercise left to the reader). tracks adding a ESM build to this project.


    First, import the library:

    // In the browser:
    import { setDayStartInterval } from 'day-start-interval';
    // In Node:
    const { setDayStartInterval } = require('day-start-interval');

    Then, determine the user's time zone. The library can't determine when the day starts without this information: as of this writing, it is 2:06pm on September 18th in San Francisco, but already 6:36am on September 19th in Adelaide, Australia!

    Client-side, you can use moment-timezone and use to figure this out:

    import moment from 'moment-timezone';
    const tz =;

    If you wish to use this library server-side, you can determine the time zone client-side and then pass that to the server (exercise left to the reader).


    setDayStartInterval(() => {
      console.log("It's a new day!");
    }, { tz });

    Does this library expect your code to actually run for more than 24 hours?

    I mean… it might, right? Some users keep tabs open forever, and you might even expect them to do so if you're making some sort of dashboard for them.

    And if the client is alive for more than 24 hours, than a corresponding websocket process might live for more than 24 hours too. (Theoretically—you should make sure that your client can gracefully reconnect if your websocket servers cycle, the network connection drops, etc!)

    But, it's totally possible for setDayStartInterval to fire much sooner than 24 hours, depending on when you call it—if you call it at 11:59pm, then it will fire for the first time in just one minute.

    Error handling

    As with setInterval, if func throws, that exception will not be caught. You are responsible for adding your own try-catch inside func if needed. If func throws, this library does not make any guarantees about whether the interval will continue to run.


    We welcome bug reports and feature suggestions, as well as contributions!

    Please add tests for any changes. You can run the tests continuously as you work by doing env WATCH=true npm run test.


    npm i day-start-interval

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