Cleanup util time tests

- Use now variable instead of walking the clock.
- Use iso format instead of seconds.
- Pass a Date object instead of an iso format to time.<methods>so
the methods off the Date object can be mocked in order to avoid
timezone specific tests. Since methods like getFullYear output the
year in whatever timezone the operating system is set to, these
methods must be mocked/mapped to their UTC equivalents when testing
such as getUTCFullYear.
- Remove offseting of time based on timezone as this is now covered
by mocking the getFullYear method and not needed for the decayInterval
as that only cares about time deltas.

src/sidebar/components/timestamp.js

@@ -17,10 +17,10 @@ function Timestamp({ className, href, timestamp }) {
   const [now, setNow] = useState(new Date());
 
   // Fuzzy, relative timestamp (eg. '6 days ago')
-  const relativeTimestamp = useMemo(() => toFuzzyString(timestamp, now), [
-    timestamp,
-    now,
-  ]);
+  const relativeTimestamp = useMemo(
+    () => toFuzzyString(timestamp ? new Date(timestamp) : null, now),
+    [timestamp, now]
+  );
 
   // Absolute timestamp (eg. 'Tue 22nd Dec 2015, 16:00')
   const absoluteTimestamp = useMemo(() => formatDate(new Date(timestamp)), [

src/sidebar/util/time.js

@@ -55,8 +55,14 @@ function nHr(date, now) {
 
 // Cached DateTimeFormat instances,
 // because instantiating a DateTimeFormat is expensive.
-const formatters = {};
+let formatters = {};
 
+/**
+ * Clears the cache of formatters.
+ */
+function clearFormatters() {
+  formatters = {};
+}
 /**
  * Efficiently return `date` formatted with `options`.
  *
@@ -139,10 +145,15 @@ const BREAKPOINTS = [
   },
 ];
 
+/**
+ * Returns a dict that describes how to format the date based on the delta
+ * between date and now.
+ *
+ * @param {Date} date - The date to consider as the timestamp to format.
+ * @param {Date} now - The date to consider as the current time.
+ * @return {breakpoint} A dict that describes how to format the date.
+ */
 function getBreakpoint(date, now) {
-  // Turn the given ISO 8601 string into a Date object.
-  date = new Date(date);
-
   let breakpoint;
   for (let i = 0; i < BREAKPOINTS.length; i++) {
     breakpoint = BREAKPOINTS[i];
@@ -154,12 +165,12 @@ function getBreakpoint(date, now) {
   return null;
 }
 
-function nextFuzzyUpdate(date) {
+function nextFuzzyUpdate(date, now) {
   if (!date) {
     return null;
   }
 
-  let secs = getBreakpoint(date, new Date()).nextUpdate;
+  let secs = getBreakpoint(date, now).nextUpdate;
 
   if (secs === null) {
     return null;
@@ -183,12 +194,15 @@ function nextFuzzyUpdate(date) {
  * This can be used to refresh parts of a UI whose
  * update frequency depends on the age of a timestamp.
  *
+ * @param {String} date - An ISO 8601 date string timestamp to format.
+ * @param {Date} callback - A callback function to call when the timestamp changes.
  * @return {Function} A function that cancels the automatic refresh.
  */
 function decayingInterval(date, callback) {
   let timer;
+  const timeStamp = date ? new Date(date) : null;
   const update = function() {
-    const fuzzyUpdate = nextFuzzyUpdate(date);
+    const fuzzyUpdate = nextFuzzyUpdate(timeStamp, new Date());
     if (fuzzyUpdate === null) {
       return;
     }
@@ -208,18 +222,20 @@ function decayingInterval(date, callback) {
 /**
  * Formats a date as a string relative to the current date.
  *
- * @param {number} date - The absolute timestamp to format.
- * @param {Date} [now] - The date to consider as the current time.
+ * @param {Date} date - The date to consider as the timestamp to format.
+ * @param {Date} now - The date to consider as the current time.
+ * @param {Object} Intl - JS internationalization API implementation.
  * @return {string} A 'fuzzy' string describing the relative age of the date.
  */
-function toFuzzyString(date, now = new Date(), Intl) {
+function toFuzzyString(date, now, Intl) {
   if (!date) {
     return '';
   }
-  return getBreakpoint(date, now).format(new Date(date), now, Intl);
+  return getBreakpoint(date, now).format(date, now, Intl);
 }
 
 module.exports = {
+  clearFormatters: clearFormatters,
   decayingInterval: decayingInterval,
   nextFuzzyUpdate: nextFuzzyUpdate,
   toFuzzyString: toFuzzyString,