diff --git a/web/js/datepicker.js.php b/web/js/datepicker.js.php
index 6deb9c4489..5f6c272642 100644
--- a/web/js/datepicker.js.php
+++ b/web/js/datepicker.js.php
@@ -38,10 +38,25 @@ function iPadMobileFix() {
 ?>
 function getISODate(year, month, day)
 {
+  <?php // toISOString() converts to UTC, so make the date a UTC date ?>
   var date = new Date(Date.UTC(year, month, day));
   return date.toISOString().split('T')[0];
 }
 
+
+// Given a JavaScript Date object returns a date string in YYYY-MM-DD
+// format.  (Note that toISOString() returns a date in UTC time).
+function getLocalISODateString(date)
+{
+  var month = (date.getMonth() + 1).toString().padStart(2, '0');
+  var day = date.getDate().toString().padStart(2, '0');
+  var year = date.getFullYear().toString();
+
+  return [year, month, day].join('-');
+}
+
+
+
 <?php
 // Functions to find the start and end dates of a week and month given a
 // date in YYYY-MM-DD format.
@@ -51,30 +66,39 @@ function getISODate(year, month, day)
 ?>
 
 function weekStart(date, weekStarts) {
-  var d = new Date(date);
+  <?php
+  // Need to add a time to make sure the date is interpreted as local rather than UTC.
+  // "When the time zone offset is absent, date-only forms are interpreted as a UTC time and
+  // date-time forms are interpreted as local time. This is due to a historical spec error
+  // that was not consistent with ISO 8601 but could not be changed due to web compatibility."
+  ?>
+  var d = new Date(date + "T00:00:00");
   var diff = d.getDay() - weekStarts;
   if (diff < 0)
   {
     diff += 7;
   }
   d.setDate(d.getDate() - diff);
-  return d.toISOString().split('T')[0];
+  return getLocalISODateString(d);
 }
 
 function weekEnd(date, weekStarts) {
-  var d = new Date(weekStart(date, weekStarts));
+  <?php // Need to add a time to make sure the date is interpreted as local rather than UTC. ?>
+  var d = new Date(weekStart(date, weekStarts) + "T00:00:00");
   d.setDate(d.getDate() + 6);
-  return d.toISOString().split('T')[0];
+  return getLocalISODateString(d);
 }
 
 function monthStart(date) {
-  var d = new Date(date);
+  <?php // Need to add a time to make sure the date is interpreted as local rather than UTC. ?>
+  var d = new Date(date + "T00:00:00");
   d.setDate(1);
-  return d.toISOString().split('T')[0];
+  return getLocalISODateString(d);
 }
 
 function monthEnd(date) {
-  var d = new Date(date);
+  <?php // Need to add a time to make sure the date is interpreted as local rather than UTC. ?>
+  var d = new Date(date + "T00:00:00");
   <?php
   // Set the date to the first of the month, because otherwise we will
   // advance by two months when incrementing the month below if we're at the
@@ -83,7 +107,7 @@ function monthEnd(date) {
   d.setDate(1);
   d.setMonth(d.getMonth() + 1);
   d.setDate(0);
-  return d.toISOString().split('T')[0];
+  return getLocalISODateString(d);
 }
 
 
@@ -91,17 +115,18 @@ function monthEnd(date) {
 // excluding hidden days.
 function datesInRange(startDate, endDate, excludeHiddenDays) {
   var result=[];
-  var e=new Date(endDate);
+  <?php // Need to add a time to make sure the date is interpreted as local rather than UTC. ?>
+  var e=new Date(endDate + "T00:00:00");
   var hiddenDays = [<?php echo implode(',', $hidden_days)?>];
 
   <?php // dates can be compared using > and < but not == or === ?>
-  for (var d=new Date(startDate); !(d>e); d.setDate(d.getDate()+1))
+  for (var d=new Date(startDate + "T00:00:00"); !(d>e); d.setDate(d.getDate()+1))
   {
     if (excludeHiddenDays && (hiddenDays.indexOf(d.getDay()) >= 0))
     {
       continue;
     }
-    result.push(d.toISOString().split('T')[0]);
+    result.push(getLocalISODateString(d));
   }
 
   return result;