diff --git a/bower.json b/bower.json
index 58e62a0769..8d0e266c2f 100644
--- a/bower.json
+++ b/bower.json
@@ -18,6 +18,7 @@
     "bootstrap": "~3.3.4",
     "font-awesome": "~4.3.0",
     "moment": "~2.10.2",
+    "moment-timezone": "~0.5.0",
     "jshint": "~2.9.0",
     "lightbox2": "~2.8.1",
     "rxjs": "~4.0.6",
diff --git a/client/commonFramework/show-completion.js b/client/commonFramework/show-completion.js
index 5080efd629..752938993a 100644
--- a/client/commonFramework/show-completion.js
+++ b/client/commonFramework/show-completion.js
@@ -1,6 +1,7 @@
 window.common = (function(global) {
   const {
     $,
+    moment,
     ga = (() => {}),
       common = { init: [] }
   } = global;
@@ -51,7 +52,8 @@ window.common = (function(global) {
         name: common.challengeName,
         completedWith: didCompleteWith,
         challengeType: common.challengeType,
-        solution
+        solution,
+        timezone: moment.tz.guess()
       };
 
       $.post('/completed-challenge/', data, function(res) {
diff --git a/common/models/user.json b/common/models/user.json
index 66a42f437b..ad68b5c2fb 100644
--- a/common/models/user.json
+++ b/common/models/user.json
@@ -173,6 +173,9 @@
     },
     "tshirtVote": {
       "type": "number"
+    },
+    "timezone": {
+      "type": "string"
     }
   },
   "validations": [],
diff --git a/gulpfile.js b/gulpfile.js
index 5069c1bca0..4b87263ad1 100644
--- a/gulpfile.js
+++ b/gulpfile.js
@@ -99,6 +99,7 @@ var paths = {
     'public/bower_components/bootstrap/dist/js/bootstrap.min.js',
     'public/bower_components/d3/d3.min.js',
     'public/bower_components/moment/min/moment.min.js',
+    'public/bower_components/moment-timezone/builds/moment-timezone-with-data.min.js',
     'public/bower_components/mousetrap/mousetrap.min.js',
     'public/bower_components/lightbox2/dist/js/lightbox.min.js',
     'public/bower_components/rxjs/dist/rx.all.min.js'
diff --git a/package.json b/package.json
index 8ec11c6ca5..bdb047d58b 100644
--- a/package.json
+++ b/package.json
@@ -94,6 +94,7 @@
     "merge-stream": "^1.0.0",
     "method-override": "^2.3.0",
     "moment": "^2.10.2",
+    "moment-timezone": "^0.5.0",
     "mongodb": "^2.0.33",
     "morgan": "^1.6.1",
     "node-uuid": "^1.4.3",
diff --git a/server/boot/challenge.js b/server/boot/challenge.js
index df0ad21d8a..bf26a8a78b 100644
--- a/server/boot/challenge.js
+++ b/server/boot/challenge.js
@@ -505,7 +505,8 @@ module.exports = function(app) {
       id,
       name,
       challengeType,
-      solution
+      solution,
+      timezone
     } = req.body;
 
     const { alreadyCompleted } = updateUserProgress(
@@ -521,6 +522,10 @@ module.exports = function(app) {
       }
     );
 
+    if (timezone && (!req.user.timezone || req.user.timezone != timezone)) {
+      req.user.timezone = timezone;
+    }
+
     let user = req.user;
     saveUser(req.user)
       .subscribe(
diff --git a/server/boot/user.js b/server/boot/user.js
index a4924fe6d8..e4c9061d35 100644
--- a/server/boot/user.js
+++ b/server/boot/user.js
@@ -1,6 +1,6 @@
 import _ from 'lodash';
 import dedent from 'dedent';
-import moment from 'moment';
+import moment from 'moment-timezone';
 import { Observable } from 'rx';
 import debugFactory from 'debug';
 
@@ -38,6 +38,8 @@ const certText = {
   [certTypes.fullStack]: 'Full Stack Certified'
 };
 
+const dateFormat = 'MMM DD, YYYY';
+
 function replaceScriptTags(value) {
   return value
     .replace(/<script>/gi, 'fccss')
@@ -183,6 +185,12 @@ module.exports = function(app) {
         }
         profileUser = profileUser.toJSON();
 
+        // timezone of signed-in account
+        // to show all date related components
+        // using signed-in account's timezone
+        // not of the profile she is viewing
+        const timezone = req.user.timezone || 'UTC';
+
         var cals = profileUser
           .progressTimestamps
           .map(objOrNum => {
@@ -192,8 +200,8 @@ module.exports = function(app) {
           })
           .sort();
 
-        profileUser.currentStreak = calcCurrentStreak(cals);
-        profileUser.longestStreak = calcLongestStreak(cals);
+        profileUser.currentStreak = calcCurrentStreak(cals, timezone);
+        profileUser.longestStreak = calcLongestStreak(cals, timezone);
 
         const data = profileUser
           .progressTimestamps
@@ -223,9 +231,20 @@ module.exports = function(app) {
             +challenge.challengeType === 4;
         }
 
-        const completedChallenges = profileUser.completedChallenges.filter(
-          ({ name }) => typeof name === 'string'
-        );
+        const completedChallenges = profileUser.completedChallenges
+          .filter(({ name }) => typeof name === 'string')
+          .map(challenge => {
+              challenge = { ...challenge };
+              if (challenge.completedDate) {
+                challenge.completedDate = 
+                  moment.tz(challenge.completedDate, timezone).format(dateFormat);
+              }
+              if (challenge.lastUpdated) {
+                challenge.lastUpdated = 
+                  moment.tz(challenge.lastUpdated, timezone).format(dateFormat);
+              }
+              return challenge;
+          });
 
         const projects = completedChallenges.filter(filterProjects);
 
diff --git a/server/utils/date-utils.js b/server/utils/date-utils.js
index 870f094977..ff62b7a619 100644
--- a/server/utils/date-utils.js
+++ b/server/utils/date-utils.js
@@ -1,10 +1,10 @@
-import moment from 'moment';
+import moment from 'moment-timezone';
 
 // day count between two epochs (inclusive)
-export function dayCount([head, tail]) {
+export function dayCount([head, tail], timezone = 'UTC') {
   return Math.ceil(
-    moment(moment(head).endOf('day')).diff(
-      moment(tail).startOf('day'),
+    moment(moment(head).tz(timezone).endOf('day')).tz(timezone).diff(
+      moment(tail).tz(timezone).startOf('day'),
       'days',
       true)
     );
diff --git a/server/utils/user-stats.js b/server/utils/user-stats.js
index 324c4761f5..e7ac94dc27 100644
--- a/server/utils/user-stats.js
+++ b/server/utils/user-stats.js
@@ -1,12 +1,12 @@
-import moment from 'moment';
+import moment from 'moment-timezone';
 import { dayCount } from '../utils/date-utils';
 
 const daysBetween = 1.5;
 
-export function calcCurrentStreak(cals) {
+export function calcCurrentStreak(cals, timezone = 'UTC') {
   const revCals = cals.slice().reverse();
 
-  if (dayCount([moment(), revCals[0]]) > daysBetween) {
+  if (dayCount([moment().tz(timezone), revCals[0]], timezone) > daysBetween) {
     return 0;
   }
 
@@ -16,7 +16,7 @@ export function calcCurrentStreak(cals) {
       const before = revCals[index === 0 ? 0 : index - 1];
       if (
         !streakBroken &&
-        moment(before).diff(cal, 'days', true) < daysBetween
+        moment(before).tz(timezone).diff(cal, 'days', true) < daysBetween
         ) {
         return index;
       }
@@ -25,22 +25,22 @@ export function calcCurrentStreak(cals) {
     }, 0);
 
   const lastTimestamp = revCals[lastDayInStreak];
-  return dayCount([moment(), lastTimestamp]);
+  return dayCount([moment().tz(timezone), lastTimestamp], timezone);
 }
 
-export function calcLongestStreak(cals) {
+export function calcLongestStreak(cals, timezone = 'UTC') {
   let tail = cals[0];
   const longest = cals.reduce((longest, head, index) => {
     const last = cals[index === 0 ? 0 : index - 1];
     // is streak broken
-    if (moment(head).diff(last, 'days', true) > daysBetween) {
+    if (moment(head).tz(timezone).diff(last, 'days', true) > daysBetween) {
       tail = head;
     }
-    if (dayCount(longest) < dayCount([head, tail])) {
+    if (dayCount(longest, timezone) < dayCount([head, tail], timezone)) {
       return [head, tail];
     }
     return longest;
   }, [cals[0], cals[0]]);
 
-  return dayCount(longest);
+  return dayCount(longest, timezone);
 }
diff --git a/server/views/account/show.jade b/server/views/account/show.jade
index f7cf7c4e80..8b898ae195 100644
--- a/server/views/account/show.jade
+++ b/server/views/account/show.jade
@@ -152,8 +152,8 @@ block content
                           for challenge in algos
                               tr
                                   td.col-xs-5.hidden-xs= removeOldTerms(challenge.name)
-                                  td.col-xs-2.hidden-xs= challenge.completedDate ? moment(challenge.completedDate, 'x').format("MMM DD, YYYY") : 'Not Available'
-                                  td.col-xs-2.hidden-xs= challenge.lastUpdated ? moment(challenge.lastUpdated, 'x').format("MMM DD, YYYY") : ''
+                                  td.col-xs-2.hidden-xs= challenge.completedDate ? challenge.completedDate : 'Not Available'
+                                  td.col-xs-2.hidden-xs= challenge.lastUpdated ? challenge.lastUpdated : ''
                                   td.col-xs-2.hidden-xs
                                     if (challenge.solution)
                                         a(href='/challenges/' + removeOldTerms(challenge.name) + '?solution=' + encodeURIComponent(encodeFcc(challenge.solution)), target='_blank') View my solution
@@ -176,8 +176,8 @@ block content
                           for challenge in challenges
                               tr
                                   td.col-xs-5.hidden-xs= removeOldTerms(challenge.name)
-                                  td.col-xs-2.hidden-xs= challenge.completedDate ?moment(challenge.completedDate, 'x').format("MMM DD, YYYY") : 'Not Available'
-                                  td.col-xs-2.hidden-xs= challenge.lastUpdated ? moment(challenge.lastUpdated, 'x').format("MMM DD, YYYY") : ''
+                                  td.col-xs-2.hidden-xs= challenge.completedDate ? challenge.completedDate : 'Not Available'
+                                  td.col-xs-2.hidden-xs= challenge.lastUpdated ? challenge.lastUpdated : ''
                                   td.col-xs-2.hidden-xs
                                     if (challenge.solution)
                                         a(href='/challenges/' + removeOldTerms(challenge.name) + '?solution=' + encodeURIComponent(encodeFcc(challenge.solution)), target='_blank') View my solution
diff --git a/test/server/utils/date-utils-test.js b/test/server/utils/date-utils-test.js
index 41a37edb89..0a2366a150 100644
--- a/test/server/utils/date-utils-test.js
+++ b/test/server/utils/date-utils-test.js
@@ -1,34 +1,44 @@
-import moment from 'moment';
+import moment from 'moment-timezone';
 
 import { dayCount } from '../../../server/utils/date-utils';
 
 let test = require('tape');
 
 test('Day count between two epochs (inclusive) calculation', function (t) {
-  t.plan(5);
+  t.plan(7);
 
   t.equal(dayCount([
-    moment("8/3/2015 3:00", "M/D/YYYY H:mm").valueOf(),
-    moment("8/3/2015 2:00", "M/D/YYYY H:mm").valueOf()
+    moment.utc("8/3/2015 3:00", "M/D/YYYY H:mm").valueOf(),
+    moment.utc("8/3/2015 2:00", "M/D/YYYY H:mm").valueOf()
   ]), 1, "should return 1 day given epochs of the same day");
 
   t.equal(dayCount([
-    moment("8/3/2015 2:00", "M/D/YYYY H:mm").valueOf(),
-    moment("8/3/2015 2:00", "M/D/YYYY H:mm").valueOf()
+    moment.utc("8/3/2015 2:00", "M/D/YYYY H:mm").valueOf(),
+    moment.utc("8/3/2015 2:00", "M/D/YYYY H:mm").valueOf()
   ]), 1, "should return 1 day given same epochs");
 
   t.equal(dayCount([
-    moment("8/4/2015 2:00", "M/D/YYYY H:mm").valueOf(),
-    moment("8/3/2015 2:00", "M/D/YYYY H:mm").valueOf()
+    moment.utc("8/4/2015 2:00", "M/D/YYYY H:mm").valueOf(),
+    moment.utc("8/3/2015 2:00", "M/D/YYYY H:mm").valueOf()
   ]), 2, "should return 2 days when there is a 24 hours difference between given dates");
 
   t.equal(dayCount([
-    moment("8/4/2015 1:00", "M/D/YYYY H:mm").valueOf(),
-    moment("8/3/2015 23:00", "M/D/YYYY H:mm").valueOf()
-  ]), 2, "should return 2 days when the diff is less than 24h but days of the month are different");
+    moment.utc("8/4/2015 1:00", "M/D/YYYY H:mm").valueOf(),
+    moment.utc("8/3/2015 23:00", "M/D/YYYY H:mm").valueOf()
+  ]), 2, "should return 2 days when the diff is less than 24h but days are different in default timezone UTC");
 
   t.equal(dayCount([
-    moment("10/27/2015 1:00", "M/D/YYYY H:mm").valueOf(),
-    moment("5/12/1982 1:00", "M/D/YYYY H:mm").valueOf()
+    moment.utc("8/4/2015 1:00", "M/D/YYYY H:mm").valueOf(),
+    moment.utc("8/3/2015 23:00", "M/D/YYYY H:mm").valueOf()
+  ], 'America/Los_Angeles'), 1, "should return 1 day when the diff is less than 24h and days are different in UTC, but given 'America/Los_Angeles' timezone");
+
+  t.equal(dayCount([
+    moment.utc("10/27/2015 1:00", "M/D/YYYY H:mm").valueOf(),
+    moment.utc("5/12/1982 1:00", "M/D/YYYY H:mm").valueOf()
   ]), 12222, "should return correct count when there is very big period");
+  
+  t.equal(dayCount([
+    moment.utc("8/4/2015 2:00", "M/D/YYYY H:mm").valueOf(),
+    moment.utc("8/3/2015 2:00", "M/D/YYYY H:mm").valueOf()
+  ], undefined), 2, "should return 2 days when there is a 24 hours difference between dates given 'undefined' timezone");
 });
diff --git a/test/server/utils/user-stats-test.js b/test/server/utils/user-stats-test.js
index f14a312138..5dcba8d4c9 100644
--- a/test/server/utils/user-stats-test.js
+++ b/test/server/utils/user-stats-test.js
@@ -1,132 +1,160 @@
-import moment from 'moment';
+import moment from 'moment-timezone';
 
 import { calcCurrentStreak, calcLongestStreak } from '../../../server/utils/user-stats';
 
 let test = require('tape');
 
 test('Current streak calculation', function (t) {
-  t.plan(7);
+  t.plan(9);
 
   t.equal(calcCurrentStreak([
-      moment(moment().subtract(1, 'hours')).valueOf()
+      moment.utc(moment.utc().subtract(1, 'hours')).valueOf()
     ]), 1, "should return 1 day when today one challenge was completed");
 
   t.equal(calcCurrentStreak([
-      moment(moment().subtract(2, 'hours')).valueOf(),
-      moment(moment().subtract(1, 'hours')).valueOf()
-    ]), 1, "should return 1 day when today more than one challenge were completed");
+      moment.utc(moment.utc().subtract(1, 'hours')).valueOf(),
+      moment.utc(moment.utc().subtract(1, 'hours')).valueOf()
+    ]), 1, "should return 1 day when today more than one challenge was completed");
 
   t.equal(calcCurrentStreak([
-      moment("9/11/2015 4:00", "M/D/YYYY H:mm").valueOf()
+      moment.utc("9/11/2015 4:00", "M/D/YYYY H:mm").valueOf()
     ]), 0, "should return 0 day when today 0 challenges were completed");
 
   t.equal(calcCurrentStreak([
-      moment(moment().subtract(1, 'days')).valueOf(),
-      moment(moment().subtract(1, 'hours')).valueOf()
+      moment.utc(moment.utc().subtract(1, 'days')).valueOf(),
+      moment.utc(moment.utc().subtract(1, 'hours')).valueOf()
     ]), 2, "should return 2 days when today and yesterday challenges were completed");
 
   t.equal(calcCurrentStreak([
-      moment("8/3/2015 2:00", "M/D/YYYY H:mm").valueOf(),
-      moment("9/11/2015 4:00", "M/D/YYYY H:mm").valueOf(),
-      moment("9/12/2015 1:00", "M/D/YYYY H:mm").valueOf(),
-      moment("9/12/2015 1:00", "M/D/YYYY H:mm").valueOf(),
-      moment("9/12/2015 2:00", "M/D/YYYY H:mm").valueOf(),
-      moment("9/12/2015 3:00", "M/D/YYYY H:mm").valueOf(),
-      moment("9/13/2015 4:00", "M/D/YYYY H:mm").valueOf(),
-      moment("9/14/2015 5:00", "M/D/YYYY H:mm").valueOf(),
-      moment(moment().subtract(2, 'days')).valueOf(),
-      moment(moment().subtract(1, 'days')).valueOf(),
-      moment(moment().subtract(1, 'hours')).valueOf()
+      moment.utc("8/3/2015 2:00", "M/D/YYYY H:mm").valueOf(),
+      moment.utc("9/11/2015 4:00", "M/D/YYYY H:mm").valueOf(),
+      moment.utc("9/12/2015 1:00", "M/D/YYYY H:mm").valueOf(),
+      moment.utc("9/12/2015 1:00", "M/D/YYYY H:mm").valueOf(),
+      moment.utc("9/12/2015 2:00", "M/D/YYYY H:mm").valueOf(),
+      moment.utc("9/12/2015 3:00", "M/D/YYYY H:mm").valueOf(),
+      moment.utc("9/13/2015 4:00", "M/D/YYYY H:mm").valueOf(),
+      moment.utc("9/14/2015 5:00", "M/D/YYYY H:mm").valueOf(),
+      moment.utc(moment.utc().subtract(2, 'days')).valueOf(),
+      moment.utc(moment.utc().subtract(1, 'days')).valueOf(),
+      moment.utc(moment.utc().subtract(1, 'hours')).valueOf()
     ]), 3, "should return 3 when today and for two days before user was activity");
 
   t.equal(calcCurrentStreak([
-      moment(moment().subtract(37, 'hours')).valueOf(),
-      moment(moment().subtract(1, 'hours')).valueOf()
+      moment.utc(moment.utc().subtract(37, 'hours')).valueOf(),
+      moment.utc(moment.utc().subtract(1, 'hours')).valueOf()
     ]), 1, "should return 1 when between todays challenge completion and yesterdays there is a 1.5 day (36 hours) long break");  
 
+  t.ok(calcCurrentStreak([
+      moment.utc(moment.utc().subtract(35, 'hours')).valueOf(),
+      moment.utc(moment.utc().subtract(1, 'hours')).valueOf()
+    ]) >= 2, "should return not less than 2 days when between todays challenge completion and yesterdays there is less than 1.5 day (36 hours) long break");  
+
+    t.equal(calcCurrentStreak([
+      moment.utc(moment.utc().subtract(1, 'hours')).valueOf(),
+      moment.utc(moment.utc().subtract(1, 'hours')).valueOf()
+    ], undefined), 1, "should return correct count in default timezone UTC given 'undefined' timezone");
+    
   t.equal(calcCurrentStreak([
-      moment(moment().subtract(35, 'hours')).valueOf(),
-      moment(moment().subtract(1, 'hours')).valueOf()
-    ]), 2, "should return 2 days when between todays challenge completion and yesterdays there is less than 1.5 day (36 hours) long break");  
+      moment.utc(moment.utc().subtract(1, 'days')).valueOf(),
+      moment.utc(moment.utc().subtract(1, 'hours')).valueOf()
+    ], 'America/Los_Angeles'), 2, "should return 2 days when today and yesterday challenges were completed given 'America/Los_Angeles' timezone");
 
 });
 
 test('Longest streak calculation', function (t) {
-  t.plan(9);
+  t.plan(12);
 
   t.equal(calcLongestStreak([
-      moment("9/12/2015 4:00", "M/D/YYYY H:mm").valueOf()
+      moment.utc("9/12/2015 4:00", "M/D/YYYY H:mm").valueOf()
     ]), 1, "should return 1 when there is the only one one-day-long streak available");
 
   t.equal(calcLongestStreak([
-      moment("9/11/2015 4:00", "M/D/YYYY H:mm").valueOf(),
-      moment("9/12/2015 2:00", "M/D/YYYY H:mm").valueOf(),
-      moment("9/13/2015 3:00", "M/D/YYYY H:mm").valueOf(),
-      moment("9/14/2015 1:00", "M/D/YYYY H:mm").valueOf()
+      moment.utc("9/11/2015 4:00", "M/D/YYYY H:mm").valueOf(),
+      moment.utc("9/12/2015 2:00", "M/D/YYYY H:mm").valueOf(),
+      moment.utc("9/13/2015 3:00", "M/D/YYYY H:mm").valueOf(),
+      moment.utc("9/14/2015 1:00", "M/D/YYYY H:mm").valueOf()
     ]), 4, "should return 4 when there is the only one more-than-one-days-long streak available");
 
   t.equal(calcLongestStreak([
-      moment(moment().subtract(1, 'hours')).valueOf()
+      moment.utc(moment.utc().subtract(1, 'hours')).valueOf()
     ]), 1, "should return 1 when there is only one one-day-long streak and it is today");
 
   t.equal(calcLongestStreak([
-      moment(moment().subtract(1, 'days')).valueOf(),
-      moment(moment().subtract(1, 'hours')).valueOf()
+      moment.utc(moment.utc().subtract(1, 'days')).valueOf(),
+      moment.utc(moment.utc().subtract(1, 'hours')).valueOf()
     ]), 2, "should return 2 when yesterday and today makes longest streak");
 
   t.equal(calcLongestStreak([
-      moment("8/3/2015 2:00", "M/D/YYYY H:mm").valueOf(),
-      moment("9/11/2015 4:00", "M/D/YYYY H:mm").valueOf(),
-      moment("9/12/2015 2:00", "M/D/YYYY H:mm").valueOf(),
-      moment("10/4/2015 1:00", "M/D/YYYY H:mm").valueOf(),
-      moment("10/5/2015 5:00", "M/D/YYYY H:mm").valueOf(),
-      moment("10/6/2015 4:00", "M/D/YYYY H:mm").valueOf(),
-      moment("10/7/2015 5:00", "M/D/YYYY H:mm").valueOf(),
-      moment("11/3/2015 2:00", "M/D/YYYY H:mm").valueOf()
+      moment.utc("8/3/2015 2:00", "M/D/YYYY H:mm").valueOf(),
+      moment.utc("9/11/2015 4:00", "M/D/YYYY H:mm").valueOf(),
+      moment.utc("9/12/2015 2:00", "M/D/YYYY H:mm").valueOf(),
+      moment.utc("10/4/2015 1:00", "M/D/YYYY H:mm").valueOf(),
+      moment.utc("10/5/2015 5:00", "M/D/YYYY H:mm").valueOf(),
+      moment.utc("10/6/2015 4:00", "M/D/YYYY H:mm").valueOf(),
+      moment.utc("10/7/2015 5:00", "M/D/YYYY H:mm").valueOf(),
+      moment.utc("11/3/2015 2:00", "M/D/YYYY H:mm").valueOf()
     ]), 4, "should return 4 when there is a month long break");
 
    t.equal(calcLongestStreak([
-      moment("8/3/2015 2:00", "M/D/YYYY H:mm").valueOf(),
-      moment("9/11/2015 4:00", "M/D/YYYY H:mm").valueOf(),
-      moment("9/12/2015 1:00", "M/D/YYYY H:mm").valueOf(),
-      moment(moment("9/12/2015 1:00", "M/D/YYYY H:mm").add(37, 'hours')).valueOf(),
-      moment("9/14/2015 22:00", "M/D/YYYY H:mm").valueOf(),
-      moment("9/15/2015 4:00", "M/D/YYYY H:mm").valueOf(),
-      moment("10/3/2015 2:00", "M/D/YYYY H:mm").valueOf()
+      moment.utc("8/3/2015 2:00", "M/D/YYYY H:mm").valueOf(),
+      moment.utc("9/11/2015 4:00", "M/D/YYYY H:mm").valueOf(),
+      moment.utc("9/12/2015 1:00", "M/D/YYYY H:mm").valueOf(),
+      moment.utc(moment.utc("9/12/2015 1:00", "M/D/YYYY H:mm").add(37, 'hours')).valueOf(),
+      moment.utc("9/14/2015 22:00", "M/D/YYYY H:mm").valueOf(),
+      moment.utc("9/15/2015 4:00", "M/D/YYYY H:mm").valueOf(),
+      moment.utc("10/3/2015 2:00", "M/D/YYYY H:mm").valueOf()
     ]), 3, "should return 3 when there is a more than 1.5 days long break of (36 hours)");
 
    t.equal(calcLongestStreak([
-      moment("8/3/2015 2:00", "M/D/YYYY H:mm").valueOf(),
-      moment("9/11/2015 4:00", "M/D/YYYY H:mm").valueOf(),
-      moment("9/12/2015 1:00", "M/D/YYYY H:mm").valueOf(),
-      moment("9/12/2015 1:00", "M/D/YYYY H:mm").valueOf(),
-      moment("9/12/2015 2:00", "M/D/YYYY H:mm").valueOf(),
-      moment("9/12/2015 3:00", "M/D/YYYY H:mm").valueOf(),
-      moment("9/13/2015 4:00", "M/D/YYYY H:mm").valueOf(),
-      moment("9/14/2015 5:00", "M/D/YYYY H:mm").valueOf(),
-      moment(moment().subtract(2, 'days')).valueOf(),
-      moment(moment().subtract(1, 'days')).valueOf(),
-      moment().valueOf()
+      moment.utc("8/3/2015 2:00", "M/D/YYYY H:mm").valueOf(),
+      moment.utc("9/11/2015 4:00", "M/D/YYYY H:mm").valueOf(),
+      moment.utc("9/12/2015 1:00", "M/D/YYYY H:mm").valueOf(),
+      moment.utc("9/12/2015 1:00", "M/D/YYYY H:mm").valueOf(),
+      moment.utc("9/12/2015 2:00", "M/D/YYYY H:mm").valueOf(),
+      moment.utc("9/12/2015 3:00", "M/D/YYYY H:mm").valueOf(),
+      moment.utc("9/13/2015 4:00", "M/D/YYYY H:mm").valueOf(),
+      moment.utc("9/14/2015 5:00", "M/D/YYYY H:mm").valueOf(),
+      moment.utc(moment.utc().subtract(2, 'days')).valueOf(),
+      moment.utc(moment.utc().subtract(1, 'days')).valueOf(),
+      moment.utc().valueOf()
     ]), 4, "should return 4 when the longest streak consist of several same day timestamps");
 
   t.equal(calcLongestStreak([
-      moment("8/3/2015 2:00", "M/D/YYYY H:mm").valueOf(),
-      moment("9/11/2015 4:00", "M/D/YYYY H:mm").valueOf(),
-      moment("9/12/2015 1:00", "M/D/YYYY H:mm").valueOf(),
-      moment("9/13/2015 4:00", "M/D/YYYY H:mm").valueOf(),
-      moment("9/14/2015 5:00", "M/D/YYYY H:mm").valueOf(),
-      moment("10/11/2015 4:00", "M/D/YYYY H:mm").valueOf(),
-      moment("10/12/2015 1:00", "M/D/YYYY H:mm").valueOf(),
-      moment("10/13/2015 4:00", "M/D/YYYY H:mm").valueOf(),
-      moment("10/14/2015 5:00", "M/D/YYYY H:mm").valueOf()
+      moment.utc("8/3/2015 2:00", "M/D/YYYY H:mm").valueOf(),
+      moment.utc("9/11/2015 4:00", "M/D/YYYY H:mm").valueOf(),
+      moment.utc("9/12/2015 1:00", "M/D/YYYY H:mm").valueOf(),
+      moment.utc("9/13/2015 4:00", "M/D/YYYY H:mm").valueOf(),
+      moment.utc("9/14/2015 5:00", "M/D/YYYY H:mm").valueOf(),
+      moment.utc("10/11/2015 4:00", "M/D/YYYY H:mm").valueOf(),
+      moment.utc("10/12/2015 1:00", "M/D/YYYY H:mm").valueOf(),
+      moment.utc("10/13/2015 4:00", "M/D/YYYY H:mm").valueOf(),
+      moment.utc("10/14/2015 5:00", "M/D/YYYY H:mm").valueOf()
     ]), 4, "should return 4 when there are several longest streaks (same length)");
 
   let cals = [];
   const n = 100;
   for (var i = 0; i < n; i++) {
-    cals.push(moment(moment().subtract(i, 'days')).valueOf());
+    cals.push(moment.utc(moment.utc().subtract(i, 'days')).valueOf());
   }
   cals.sort();
   t.equal(calcLongestStreak(cals), n, "should return correct longest streak when there is a very long period");
-   
+
+  t.equal(calcLongestStreak([
+      moment.utc(moment.utc().subtract(1, 'days')).valueOf(),
+      moment.utc(moment.utc().subtract(1, 'hours')).valueOf()
+    ], undefined), 2, "should return correct longest streak in default timezone UTC given 'undefined' timezone");
+
+  t.equal(calcLongestStreak([
+      moment.utc("9/11/2015 4:00", "M/D/YYYY H:mm").valueOf(),
+      moment.utc("9/12/2015 2:00", "M/D/YYYY H:mm").valueOf(),
+      moment.utc("9/13/2015 3:00", "M/D/YYYY H:mm").valueOf(),
+      moment.utc("9/14/2015 1:00", "M/D/YYYY H:mm").valueOf()
+    ], 'America/Los_Angeles'), 4, "should return 4 when there is the only one more-than-one-days-long streak available given 'America/Los_Angeles' timezone");
+
+  t.equal(calcLongestStreak([
+      moment.utc("9/11/2015 23:00", "M/D/YYYY H:mm").valueOf(),
+      moment.utc("9/12/2015 2:00", "M/D/YYYY H:mm").valueOf(),
+      moment.utc("9/13/2015 2:00", "M/D/YYYY H:mm").valueOf(),
+      moment.utc("9/14/2015 1:00", "M/D/YYYY H:mm").valueOf()
+    ], 'America/Los_Angeles'), 3, "should return 3 when longest streak is 3 in given 'America/Los_Angeles' timezone (but would be different in default timezone UTC)");
 });