fiznool · February 5, 2015 10:27 · Feb 5, 2015
diff --git a/date-duration.js b/date-duration.js
@@ -0,0 +1,47 @@
+Date.prototype.duration = function(granularity) {
+  if(!granularity) {
+    granularity = 's';
+  }
+
+  var units = [{
+    enabled: /^[wdhms]$/.test(granularity),
+    singular: 'week',
+    plural: 'weeks',
+    inMillis: 7 * 24 * 60 * 60 * 1000
+  }, {
+    enabled: /^[dhms]$/.test(granularity),
+    singular: 'day',
+    plural: 'days',
+    inMillis: 24 * 60 * 60 * 1000
+  }, {
+    enabled: /^[hms]$/.test(granularity),
+    singular: 'hour',
+    plural: 'hours',
+    inMillis: 60 * 60 * 1000
+  }, {
+    enabled: /^[ms]$/.test(granularity),
+    singular: 'minute',
+    plural: 'minutes',
+    inMillis: 60 * 1000
+  }, {
+    enabled: /^[s]$/.test(granularity),
+    singular: 'second',
+    plural: 'seconds',
+    inMillis: 1000
+  }];
+
+  var duration = [];
+  var millis = this.getTime() - Date.now();
+
+  units.forEach(function(unit) {
+    if(unit.enabled) {
+      var val = Math.floor(millis / unit.inMillis);
+      if(val > 0) {
+        duration.push(val + ' ' + (val > 1 ? unit.plural : unit.singular));
+        millis = millis - (val * unit.inMillis);
+      }
+    }
+  });
+
+  return duration.join(', ');
+};
diff --git a/date-duration.spec.js b/date-duration.spec.js
@@ -0,0 +1,80 @@
+describe('Date duration', function() {
+  beforeEach(function() {
+    // Ensure date under test equals the relative date
+    spyOn(Date, 'now').and.returnValue(0);
+  });
+
+  it('should return a duration', function() {
+    var oneSecond = new Date(1000);
+    expect(oneSecond.duration()).toBe('1 second');
+
+    var oneMinute = new Date(60 * 1000);
+    expect(oneMinute.duration()).toBe('1 minute');
+
+    var oneHour = new Date(60* 60 * 1000);
+    expect(oneHour.duration()).toBe('1 hour');
+
+    var oneDay = new Date(24 * 60 * 60 * 1000);
+    expect(oneDay.duration()).toBe('1 day');
+
+    var oneWeek = new Date(7 * 24 * 60 * 60 * 1000);
+    expect(oneWeek.duration()).toBe('1 week');
+  });
+
+  it('should pluralise correctly', function() {
+    var twoSecond = new Date(2 * 1000);
+    expect(twoSecond.duration()).toBe('2 seconds');
+
+    var twoMinute = new Date(2 * 60 * 1000);
+    expect(twoMinute.duration()).toBe('2 minutes');
+
+    var twoHour = new Date(2 * 60* 60 * 1000);
+    expect(twoHour.duration()).toBe('2 hours');
+
+    var twoDay = new Date(2 * 24 * 60 * 60 * 1000);
+    expect(twoDay.duration()).toBe('2 days');
+
+    var twoWeek = new Date(2 * 7 * 24 * 60 * 60 * 1000);
+    expect(twoWeek.duration()).toBe('2 weeks');
+  });
+
+  it('should combine duration units', function() {
+    var fiveMinutesThirtySeconds = new Date(
+      (5 * 60 * 1000) + (30 * 1000));
+    expect(fiveMinutesThirtySeconds.duration())
+      .toBe('5 minutes, 30 seconds');
+
+    var oneDayFourHoursFifteenMinutes = new Date(
+      (24 * 60 * 60 * 1000) +
+      (4 * 60 * 60 * 1000) +
+      (15 * 60 * 1000));
+    expect(oneDayFourHoursFifteenMinutes.duration())
+      .toBe('1 day, 4 hours, 15 minutes');
+
+    var threeWeeksFourDays = new Date(
+      (3 * 7 * 24 * 60 * 60 * 1000) +
+      (4 * 24 * 60 * 60 * 1000));
+    expect(threeWeeksFourDays.duration())
+      .toBe('3 weeks, 4 days');
+  });
+
+  it('should allow granularity to be specified', function() {
+    var fiveMinutesThirtySeconds = new Date(
+      (5 * 60 * 1000) + (30 * 1000));
+    expect(fiveMinutesThirtySeconds.duration('m'))
+      .toBe('5 minutes');
+
+    var oneDayFourHoursFifteenMinutes = new Date(
+      (24 * 60 * 60 * 1000) +
+      (4 * 60 * 60 * 1000) +
+      (15 * 60 * 1000));
+    expect(oneDayFourHoursFifteenMinutes.duration('h'))
+      .toBe('1 day, 4 hours');
+
+    var threeWeeksFourDays = new Date(
+      (3 * 7 * 24 * 60 * 60 * 1000) +
+      (4 * 24 * 60 * 60 * 1000));
+    expect(threeWeeksFourDays.duration('w'))
+      .toBe('3 weeks');
+  });
+});