No idea if this actually works, but should be something like this

gistfile1.clj

(ns some-namespace
  (:require
    [om.core :as om]
    [om.dom :as dom]))

(defn time-widget
  [{:keys [msg-time] :as msg} owner]
  (reify
    om/IRenderState
    (render-state [_ _]
      (dom/time #js {:dateTime (.fromNow (js/moment msg-time))}))))

(defn messages ; or whatever your parent component is
  [msgs owner]
  (reify
    om/IWillMount
    (will-mount [_]
      (.setInterval js/window
        #(om/set-state! owner :update (.format (js/moment))) ; just in here as a dummy value that changes
        , 1000))
    om/IRenderState
    (render-state [_ {:keys [update]}]
      (om/build-all msgs time-widget {:state {:update update}}))))

;; Alternatively

(ns some-namespace
  (:require
    [om.core :as om]
    [om.dom :as dom]))

(defn messages ; or whatever your parent component is
  [msgs owner]
  (reify
    om/IWillMount
    (will-mount [_]
      (.setInterval js/window
        #(om/set-state! owner :update (.format (js/moment))) ; just in here as a dummy value that changes
        , 1000))
    om/IRenderState
    (render-state [_ {:keys [update]}]
      (for [m msgs]
        (dom/time #js {:dateTime (.fromNow (js/moment (:msg-time m)))})))))