make date dividers reasier to read

src/lib/strformat.js

@@ -16,6 +16,21 @@ export function shortDateFormat(then, locale) {
   return then.toLocaleDateString(locale, {year: 'numeric', month: 'short', day: 'numeric'});
 }
 
+// Representation a date relative to now.
+export function relativeDateFormat(then, locale) {
+  locale = locale || window.navigator.userLanguage || window.navigator.language;
+  const now = new Date();
+  const thenDays = Math.floor((then.getTime() - then.getTimezoneOffset() * 60_000) / 86_400_000);
+  const nowDays = Math.floor((now.getTime() - now.getTimezoneOffset() * 60_000) / 86_400_000);
+  const diff = thenDays - nowDays;
+  if (Math.abs(diff) < 2) {
+    // Today or yesterday.
+    return new Intl.RelativeTimeFormat(locale, { numeric: 'auto' }).format(diff, 'day')
+  }
+  // More than two days: just show the date.
+  return new Intl.DateTimeFormat(locale).format(then);
+}
+
 // Convert seconds to [hours:]minutes:seconds, i.e. 156 sec -> 2:36, 3756 sec -> 1:02:36.
 // If <code>fixedMins</code> is true, then minutes are represented by at least two digits.
 export function secondsToTime(seconds, fixedMin) {