DateTime: avoid unnecessary update (#6110)

## Issue
Part of `5834 Performance investigation`

In a homepage with 120 tiles, the lists get rendered 3 times during initial update. The sub-components are updating recursively (will investigate), so instead of 120 DateTime renders, we have 1000+ renders.

The resolved DateTime string for `timeAgo` rarely changes, and even if the string was "a few seconds ago", there's no real need to constantly update it.

## Change
Require a minimum 1-minute delta when deciding whether the component should update. Clients can change this value as needed.

## Test
- [x] Verified `shouldComponentUpdate` doesn't end up taking more time than not having it (it's in micro-second range, compared to the millisecond render).
- [x] Profiler showed no significant improvement for low number of DateTime components, but for the 120 DateTime case, almost 1/4 of a second is saved.

ui/component/dateTime/view.jsx

@@ -2,15 +2,29 @@
 import React from 'react';
 import moment from 'moment';
 
+const DEFAULT_MIN_UPDATE_DELTA_MS = 60 * 1000;
+
 type Props = {
   date?: any,
   timeAgo?: boolean,
-  formatOptions: {},
+  formatOptions?: {},
   show?: string,
-  clock24h: boolean,
+  clock24h?: boolean,
+  minUpdateDeltaMs?: number,
 };
 
-class DateTime extends React.PureComponent<Props> {
+type State = {
+  lastRenderTime: Date,
+};
+
+class DateTime extends React.Component<Props, State> {
+  constructor(props: Props) {
+    super(props);
+    this.state = {
+      lastRenderTime: new Date(),
+    };
+  }
+
   static SHOW_DATE = 'date';
   static SHOW_TIME = 'time';
 
@@ -45,6 +59,38 @@ class DateTime extends React.PureComponent<Props> {
     return __(strId, { duration });
   }
 
+  shouldComponentUpdate(nextProps: Props): boolean {
+    if (
+      moment(this.props.date).diff(moment(nextProps.date)) !== 0 ||
+      this.props.clock24h !== nextProps.clock24h ||
+      this.props.timeAgo !== nextProps.timeAgo ||
+      this.props.minUpdateDeltaMs !== nextProps.minUpdateDeltaMs ||
+      this.props.show !== nextProps.show
+    ) {
+      return true;
+    }
+
+    if (this.props.timeAgo && nextProps.timeAgo) {
+      const minUpdateDeltaMs = this.props.minUpdateDeltaMs || DEFAULT_MIN_UPDATE_DELTA_MS;
+      const prev = moment(this.state.lastRenderTime);
+      const curr = moment(new Date());
+      const deltaMs = curr.diff(prev);
+
+      if (deltaMs > minUpdateDeltaMs) {
+        return true;
+      }
+    }
+
+    return false;
+  }
+
+  componentDidUpdate() {
+    const { timeAgo } = this.props;
+    if (timeAgo) {
+      this.setState({ lastRenderTime: new Date() });
+    }
+  }
+
   render() {
     const { date, timeAgo, show, clock24h } = this.props;