Approach for an updated current datetime (“now”) in a vue-x store (Vue 2/3) or a Pinia store (Vue 3)

timer.js

const MILLISECONDS_PER_MINUTES = 1000 * 60;

const state = {
  now: (new Date()),
  intervalTimer: null,
};

const mutations = {
  now(state) {
    state.now = new Date();
  },

  setIntervalTimer(state, callback) {
    state.intervalTimer = setInterval(() => {
      if (callback) {
        callback();
      }
    }, MILLISECONDS_PER_MINUTES * 3);
  },

  clearIntervalTimer(state) {
    if (state.intervalTimer) {
      clearInterval(state.intervalTimer);
      state.intervalTimer = null
    }
  }
};

const actions = {
  pollNow({ commit, state }) {
    if (!state.intervalTimer) {
      commit("setIntervalTimer", () => commit("now"));
    }
  },

  clearPollNow({ commit }) {
    commit("clearIntervalTimer");
  },
};

const time = {
  namespaced: true,
  state,
  mutations,
  actions,
};

export default time;

useNow.js

import { onBeforeMount, onUnmounted, ref } from 'vue'

const REFRESH_INTERVAL_MS = 1000

let timer
let subscribers = 0

/** Current date and time, updated every second. */
const now = ref(new Date())

const poll = () => {
  if (!timer) {
    timer = setInterval(() => {
      now.value = new Date()
    }, REFRESH_INTERVAL_MS)
  }
}

const stopPoll = () => {
  if (timer) {
    clearInterval(timer)
    timer = null
  }
}

/**
 * Subscribe to the polling of the current date and time.
 * Once subscribed, `now` is periodically updated.
 */
const subscribe = () => {
  if (!subscribers++) {
    poll()
  }
}

/**
 * Unsubscribe of the polling of the current date and time.
 * Once unsubscribed, `now` may stop to be updated.
 */
const unsubscribe = () => {
  subscribers--

  if (!subscribers) {
    stopPoll()
  }
}

/**
 * Current date, updated every second.
 *
 * It start to update automatically on the first component to be mounted, and
 * stop updating on the last unmounted one. For non-component usages, use
 * `useNowSubscription`.
 */
export function useNow() {
  onBeforeMount(subscribe)
  onUnmounted(unsubscribe)

  return { now }
}

/**
 * Current date, updated every second.
 *
 * It requires manual (de)activation using (un)subscribe methods. Suitable in a
 * non-component environment (like a store). For components, use `useNow`.
 */
export const useNowSubscription = { subscribe, unsubscribe, now }

Hey, thanks for the feedback @cjbeattie! I forgot I made this gist. 😅

Just added a useNow composable, suitable for Vue 3 composition API.