--- a/calendar/base/src/calTimezoneService.js
+++ b/calendar/base/src/calTimezoneService.js
@@ -566,17 +566,17 @@ function guessSystemTimezone() {
             Components.utils.reportError(filepath + ": " + ex);
             return "";
         }
     }
 
     function weekday(icsDate, timezone) {
         let calDate = cal.createDateTime(icsDate);
         calDate.timezone = timezone;
-        return cal.dateTimeToJsDate(calDate).toLocaleFormat("%a");
+        return cal.dateTimeToJsDate(calDate).toLocaleString(undefined, { weekday: "short" });
     }
 
     // Try to find a tz that matches OS/JSDate timezone.  If no name match,
     // will use first of probable timezone(s) with highest score.
     let probableTZId = "floating"; // default fallback tz if no tz matches.
     let probableTZScore = 0;
     let probableTZSource = null;
 

--- a/calendar/import-export/calOutlookCSVImportExport.js
+++ b/calendar/import-export/calOutlookCSVImportExport.js
@@ -24,26 +24,26 @@ var localeEn = {
 
     valueTrue:       "True",
     valueFalse:      "False",
 
     dateRe:          /^(\d+)\/(\d+)\/(\d+)$/,
     dateDayIndex:    2,
     dateMonthIndex:  1,
     dateYearIndex:   3,
-    dateFormat:      "%m/%d/%y",
+    dateFormat:      { year: "2-digit", month: "2-digit", day: "2-digit" },
 
     timeRe:          /^(\d+):(\d+):(\d+) (\w+)$/,
     timeHourIndex:   1,
     timeMinuteIndex: 2,
     timeSecondIndex: 3,
     timeAmPmIndex:   4,
     timeAmString:    "AM",
     timePmString:    "PM",
-    timeFormat:      "%I:%M:%S %p"
+    timeFormat:      { hour: "2-digit", minute: "2-digit", second: "2-digit" }
 };
 
 var localeNl = {
     headTitle:        "Onderwerp",
     headStartDate:    "Begindatum",
     headStartTime:    "Begintijd",
     headEndDate:      "Einddatum",
     headEndTime:      "Eindtijd",
@@ -58,23 +58,21 @@ var localeNl = {
 
     valueTrue:        "Waar",
     valueFalse:       "Onwaar",
 
     dateRe:           /^(\d+)-(\d+)-(\d+)$/,
     dateDayIndex:     1,
     dateMonthIndex:   2,
     dateYearIndex:    3,
-    dateFormat:       "%d-%m-%y",
 
     timeRe:           /^(\d+):(\d+):(\d+)$/,
     timeHourIndex:    1,
     timeMinuteIndex:  2,
-    timeSecondIndex:  3,
-    timeFormat:       "%H:%M:%S"
+    timeSecondIndex:  3
 };
 
 var locales = [localeEn, localeNl];
 
 // Windows line endings, CSV files with LF only can't be read by Outlook.
 var exportLineEnding = "\r\n";
 
 // Shared functions
@@ -423,21 +421,24 @@ calOutlookCSVExporter.prototype = {
         classDescription: "Calendar Outlook CSV Exporter",
         interfaces: calOutlookCSVExporterInterfaces
     }),
 
     getFileTypes: getOutlookCsvFileTypes,
 
     exportToStream: function(aStream, aCount, aItems) {
         // Helper functions
-        function dateString(aDateTime) { return cal.dateTimeToJsDate(aDateTime).toLocaleFormat(localeEn.dateFormat); }
-        function timeString(aDateTime) { return cal.dateTimeToJsDate(aDateTime).toLocaleFormat(localeEn.timeFormat); }
+        function dateString(aDateTime) {
+            return cal.dateTimeToJsDate(aDateTime).toLocaleString("en-US", localeEn.dateFormat);
+        }
+        function timeString(aDateTime) {
+            return cal.dateTimeToJsDate(aDateTime).toLocaleString("en-US", localeEn.timeFormat);
+        }
         function txtString(aString) { return aString || ""; }
 
-        let str = "";
         let headers = [];
         // Not using a loop here, since we need to be sure the order here matches
         // with the orders the field data is added later on
         headers.push(localeEn.headTitle);
         headers.push(localeEn.headStartDate);
         headers.push(localeEn.headStartTime);
         headers.push(localeEn.headEndDate);
         headers.push(localeEn.headEndTime);
@@ -445,17 +446,17 @@ calOutlookCSVExporter.prototype = {
         headers.push(localeEn.headAlarm);
         headers.push(localeEn.headAlarmDate);
         headers.push(localeEn.headAlarmTime);
         headers.push(localeEn.headCategories);
         headers.push(localeEn.headDescription);
         headers.push(localeEn.headLocation);
         headers.push(localeEn.headPrivate);
         headers = headers.map(hdr => '"' + hdr + '"');
-        str = headers.join(",");
+        let str = headers.join(",");
         str += exportLineEnding;
         aStream.write(str, str.length);
 
         for (let item of aItems) {
             if (!cal.isEvent(item)) {
                 // XXX TODO: warn the user (once) that tasks are not supported
                 // (bug 336175)
                 continue;

--- a/calendar/resources/content/datetimepickers/datetimepickers.xml
+++ b/calendar/resources/content/datetimepickers/datetimepickers.xml
@@ -558,18 +558,18 @@
             this.value = new Date();
         }
 
         // Change the grids in the timepicker-grids for 12-hours time format.
         if (this.ampmIndex) {
             // Find the locale strings for the AM/PM prefix/suffix.
             let amTime = new Date(2000, 0, 1, 6, 12, 34);
             let pmTime = new Date(2000, 0, 1, 18, 12, 34);
-            amTime = amTime.toLocaleFormat(this.kTimeFormatString);
-            pmTime = pmTime.toLocaleFormat(this.kTimeFormatString);
+            amTime = amTime.toLocaleString(undefined, this.kTimeFormatObject);
+            pmTime = pmTime.toLocaleString(undefined, this.kTimeFormatObject);
             let amLabel = this.parseTimeRegExp.exec(amTime)[this.ampmIndex] || "AM";
             let pmLabel = this.parseTimeRegExp.exec(pmTime)[this.ampmIndex] || "PM";
 
             document.getAnonymousElementByAttribute(this, "anonid", "timepickerGrids")
                     .changeTo12HoursFormat(amLabel, pmLabel);
         }
       ]]></constructor>
 
@@ -1454,16 +1454,17 @@
            inherits="value,onchange">
     <resources>
       <stylesheet src="chrome://lightning-common/skin/datetimepickers.css"/>
     </resources>
     <implementation>
       <constructor><![CDATA[
         Components.utils.import("resource://calendar/modules/calUtils.jsm");
 
+        this.kTimeFormatObject = { hour: "numeric", minute: "2-digit" };
         this.initDateFormat();
         this.initTimeFormat();
       ]]></constructor>
 
       <property name="value"
                 onget="return this.mValue"
                 onset="this.update(val, false)"/>
 
@@ -1774,19 +1775,19 @@
           let digitsExpr = "(\\d?\\d)(\\D)?(?:(\\d\\d)(\\D)?(?:(\\d\\d)(\\D)?)?)?";
           // any letters or '.': non-digit alphanumeric, period (a.m.), or space (P M)
           let anyAmPmExpr = "(?:[^\\d\\W]|[. ])+";
           // digitsExpr has 6 captures, so index of first ampmExpr is 1, of last is 8.
           let probeTimeRegExp =
               new RegExp("^(" + anyAmPmExpr + ")?\\s?" + digitsExpr + "(" + anyAmPmExpr + ")?\\s*$");
           const PRE_INDEX = 1, HR_INDEX = 2, MIN_INDEX = 4, SEC_INDEX = 6, POST_INDEX = 8; // eslint-disable-line no-unused-vars
           let amProbeTime = new Date(2000, 0, 1, 6, 12, 34);
-          let amProbeString = amProbeTime.toLocaleFormat("%X");
+          let amProbeString = amProbeTime.toLocaleTimeString();
           let pmProbeTime = new Date(2000, 0, 1, 18, 12, 34);
-          let pmProbeString = pmProbeTime.toLocaleFormat("%X");
+          let pmProbeString = pmProbeTime.toLocaleTimeString();
           let amFormatExpr = null, pmFormatExpr = null;
           if (amProbeString != pmProbeString) {
               let amProbeArray = probeTimeRegExp.exec(amProbeString);
               let pmProbeArray = probeTimeRegExp.exec(pmProbeString);
               if (amProbeArray != null && pmProbeArray != null) {
                   if (amProbeArray[PRE_INDEX] && pmProbeArray[PRE_INDEX] &&
                       amProbeArray[PRE_INDEX] != pmProbeArray[PRE_INDEX]) {
                       this.ampmIndex = PRE_INDEX;
@@ -1853,38 +1854,27 @@
           // Must recogize "pm" directly, otherwise in "12:34pm" the "S" pattern
           // matches the "p" character so only "m" is matched as ampm suffix.)
           //
           // digitsExpr has 6 captures, so index of first ampmExpr is 1, of last is 8.
           this.parseTimeRegExp =
               new RegExp("(" + ampmExpr + ")?\\s?" + digitsExpr + "(" + ampmExpr + ")?\\s*$");
           this.amRegExp = new RegExp("^(?:" + amExpr + ")$");
           this.pmRegExp = new RegExp("^(?:" + pmExpr + ")$");
-          // build time display format that mimics "%x" format without seconds
-          let ampmSep = (pmProbeString.includes(" ") ? " " : "");
-          if (this.ampmIndex == PRE_INDEX) {
-              this.kTimeFormatString = "%p" + ampmSep + "%I:%M";
-          } else if (this.ampmIndex == POST_INDEX) {
-              this.kTimeFormatString = "%I:%M" + ampmSep + "%p";
-          } else {
-              this.kTimeFormatString = "%H:%M";
-          }
         ]]></body>
       </method>
 
       <method name="formatDate">
         <parameter name="aDate"/>
         <body><![CDATA[
-          let dateFormatter = cal.getDateFormatter();
-          return dateFormatter.formatDateShort(cal.jsDateToDateTime(aDate, cal.floating()));
+          return cal.getDateFormatter().formatDateShort(cal.jsDateToDateTime(aDate, cal.floating()));
         ]]></body>
       </method>
 
       <method name="formatTime">
         <parameter name="aValue"/>
         <body><![CDATA[
-          // remove leading zero in hours before colon in xx01:23pmxx
-          return aValue.toLocaleFormat(this.kTimeFormatString).replace(/0(\d):/, "$1:");
+          return aValue.toLocaleTimeString(undefined, this.kTimeFormatObject);
         ]]></body>
       </method>
     </implementation>
   </binding>
 </bindings>