diff --git a/libraries/src/Form/Field/CalendarField.php b/libraries/src/Form/Field/CalendarField.php
index 428ca5a236f63..900ee2851c2f5 100644
--- a/libraries/src/Form/Field/CalendarField.php
+++ b/libraries/src/Form/Field/CalendarField.php
@@ -11,8 +11,10 @@
 \defined('JPATH_PLATFORM') or die;
 
 use DateTime;
+use DateTimeImmutable;
 use Joomla\CMS\Factory;
 use Joomla\CMS\Form\FormField;
+use Joomla\CMS\HTML\HTMLHelper;
 use Joomla\CMS\Language\Text;
 use Joomla\Registry\Registry;
 
@@ -274,7 +276,8 @@ protected function getInput()
 		{
 			$tz = date_default_timezone_get();
 			date_default_timezone_set('UTC');
-			$this->value = strftime($this->format, strtotime($this->value));
+			$date = DateTimeImmutable::createFromFormat('U', strtotime($this->value));
+			$this->value = $date->format(HTMLHelper::strftimeFormatToDateFormat($this->format));
 			date_default_timezone_set($tz);
 		}
 		else
diff --git a/libraries/src/HTML/HTMLHelper.php b/libraries/src/HTML/HTMLHelper.php
index 92893b35facb0..d2984d811a456 100644
--- a/libraries/src/HTML/HTMLHelper.php
+++ b/libraries/src/HTML/HTMLHelper.php
@@ -1072,7 +1072,7 @@ public static function tooltipText($title = '', $content = '', $translate = true
 	 * @param   string  $value    The date value
 	 * @param   string  $name     The name of the text field
 	 * @param   string  $id       The id of the text field
-	 * @param   string  $format   The date format
+	 * @param   string  $format   The date format using the @deprecated strftime format parameters
 	 * @param   mixed   $attribs  Additional HTML attributes
 	 *                            The array can have the following keys:
 	 *                            readonly      Sets the readonly parameter for the input tag
@@ -1085,6 +1085,7 @@ public static function tooltipText($title = '', $content = '', $translate = true
 	 *
 	 * @since   1.5
 	 *
+	 * @note This method uses deprecated strftime format parameters for backward compatibility.
 	 */
 	public static function calendar($value, $name, $id, $format = '%Y-%m-%d', $attribs = array())
 	{
@@ -1131,7 +1132,8 @@ public static function calendar($value, $name, $id, $format = '%Y-%m-%d', $attri
 		{
 			$tz = date_default_timezone_get();
 			date_default_timezone_set('UTC');
-			$inputvalue = strftime($format, strtotime($value));
+			$date = \DateTimeImmutable::createFromFormat('U', strtotime($value));
+			$inputvalue = $date->format(self::strftimeFormatToDateFormat($format));
 			date_default_timezone_set($tz);
 		}
 		else
@@ -1284,4 +1286,63 @@ private static function checkFileOrder($first, $second)
 
 		return '';
 	}
+
+	/**
+	 * Convert strftime format to php date format as strftime is deprecated and we have
+	 * to be able to provide same backward compatibility with existing format strings.
+	 *
+	 * @param   $strftimeformat   string The format compatible with strftime.
+	 *
+	 * @return  string The format compatible with PHP's Date functions.
+	 *
+	 * @since   __DEPLOY_VERSION__
+	 *
+	 * @throws  \Exception
+	 *
+	 * @note Thanks to @relipse for https://stackoverflow.com/questions/22665959/using-php-strftime-using-date-format-string/62781773#62781773
+	 */
+	public static function strftimeFormatToDateFormat(string $strftimeformat): string
+	{
+		$unsupported = ['%U', '%V', '%C', '%g', '%G'];
+		$foundunsupported = [];
+
+		foreach ($unsupported as $unsup)
+		{
+			if (strpos($strftimeformat, $unsup) !== false)
+			{
+				$foundunsupported[] = $unsup;
+			}
+		}
+
+		if (!empty($foundunsupported))
+		{
+			throw new \Exception("Found these unsupported chars: " . implode(",", $foundunsupported) . ' in ' . $strftimeformat);
+		}
+
+		/**
+		 * It is important to note that some do not translate accurately
+		 * ie. lowercase L is supposed to convert to number with a preceding space if it is under 10,
+		 * there is no accurate conversion, so we just use 'g'
+		 */
+		$phpdateformat = str_replace(
+			['%a','%A','%d','%e','%u','%w','%W','%b','%h','%B','%m','%y','%Y', '%D', '%F', '%x', '%n', '%t', '%H', '%k', '%I', '%l', '%M', '%p', '%P',
+				// %I:%M:%S %p
+				'%r',
+				// %H:%M
+				'%R',
+				'%S',
+				// %H:%M:%S
+				'%T',
+				'%X', '%z', '%Z', '%c', '%s', '%%'
+			],
+			['D','l', 'd', 'j', 'N', 'w', 'W', 'M', 'M', 'F', 'm', 'y', 'Y', 'm/d/y', 'Y-m-d', 'm/d/y', "\n", "\t", 'H', 'G', 'h', 'g', 'i', 'A', 'a', 'h:i:s A', 'H:i', 's', 'H:i:s', 'H:i:s', 'O', 'T',
+				// Tue Feb 5 00:45:10 2009
+				'D M j H:i:s Y' ,
+				'U', '%'
+			],
+			$strftimeformat
+		);
+
+		return $phpdateformat;
+	}
 }
diff --git a/tests/Unit/Libraries/Cms/Html/HtmlHelperTest.php b/tests/Unit/Libraries/Cms/Html/HtmlHelperTest.php
new file mode 100644
index 0000000000000..2f547d7b84763
--- /dev/null
+++ b/tests/Unit/Libraries/Cms/Html/HtmlHelperTest.php
@@ -0,0 +1,61 @@
+<?php
+/**
+ * @package     Joomla.UnitTest
+ * @subpackage  HTML
+ *
+ * @copyright   (C) 2021 Open Source Matters, Inc. <https://www.joomla.org>
+ * @license     GNU General Public License version 2 or later; see LICENSE.txt
+ */
+
+namespace Joomla\Tests\Unit\Libraries\Cms\Html;
+
+use Joomla\CMS\HTML\HTMLHelper;
+use Joomla\Tests\Unit\UnitTestCase;
+
+/**
+ * Test class for HtmlHelperTest.
+ *
+ * @since  __DEPLOY_VERSION__
+ */
+class HtmlHelperTest extends UnitTestCase
+{
+	/**
+	 * @var   string   Base HTML Output with place holders
+	 *
+	 * @since  __DEPLOY_VERSION__
+	 */
+	private $template = '<div class="field-calendar">
+		<div class="input-group">
+				<input
+			type="text"
+			id="%s"
+			name="%s"
+			value="%s"
+						class="form-control"									data-alt-value="%s" autocomplete="off">
+		<button type="button" class="btn btn-primary"
+			id="%s_btn"
+			title="JLIB_HTML_BEHAVIOR_OPEN_CALENDAR"
+			data-inputfield="testId" data-button="testId_btn" data-date-format="%s" data-firstday="" data-weekend="0,6" data-today-btn="1" data-week-numbers="1" data-show-time="0" data-show-others="1" data-time24="24" data-only-months-nav="0" data-min-year="" data-max-year="" data-date-type="gregorian"		><span class="icon-calendar" aria-hidden="true"></span>
+		<span class="visually-hidden">JLIB_HTML_BEHAVIOR_OPEN_CALENDAR</span>
+		</button>
+			</div>
+</div>
+';
+	/**
+	 * Test the replacement of using deprecated strftime with Date formats
+	 *
+	 * @since  __DEPLOY_VERSION__
+	 */
+	public function testCalendar()
+	{
+		$this->assertEquals(
+			sprintf($this->template, 'testId', 'testName', 'Mar', 'Mar', 'testId', '%b'),
+			HTMLHelper::calendar('1978-03-08 06:12:12', 'testName', 'testId', '%b', [])
+		);
+
+		$this->assertEquals(
+			sprintf($this->template, 'testId', 'testName', '1978-03-08', '1978-03-08', 'testId', '%Y-%m-%d'),
+			HTMLHelper::calendar('1978-03-08 06:12:12', 'testName', 'testId', '%Y-%m-%d', [])
+		);
+	}
+}
diff --git a/tests/Unit/UnitTestCase.php b/tests/Unit/UnitTestCase.php
index 5a40be2017dd7..ec64efecda803 100644
--- a/tests/Unit/UnitTestCase.php
+++ b/tests/Unit/UnitTestCase.php
@@ -8,6 +8,17 @@
  */
 namespace Joomla\Tests\Unit;
 
+use Joomla\CMS\Application\CMSApplication;
+use Joomla\CMS\Document\Document;
+use Joomla\CMS\Document\FactoryInterface;
+use Joomla\CMS\Factory;
+use Joomla\CMS\Language\Language;
+use Joomla\CMS\WebAsset\WebAssetManager;
+use Joomla\Input\Input;
+use Joomla\Registry\Registry;
+use ReflectionClass;
+use stdClass;
+
 /**
  * Base Unit Test case for common behaviour across unit tests
  *
@@ -15,4 +26,71 @@
  */
 abstract class UnitTestCase extends \PHPUnit\Framework\TestCase
 {
+	/**
+	 * @return void
+	 *
+	 * @since  __DEPLOY_VERSION__
+	 */
+	protected function setUp(): void
+	{
+		$this->initJoomla();
+	}
+
+	/**
+	 * Sets up the minimal amount of Joomla, mocked where possible, to run the
+	 * CMS Unit Test Cases
+	 *
+	 * @return void
+	 *
+	 * @since  __DEPLOY_VERSION__
+	 */
+	protected function initJoomla(): void
+	{
+		// Start with the easy mocks.
+		$input = $this->getMockBuilder(Input::class)
+			->getMockForAbstractClass();
+		$factory = $this->createMock(Factory::class);
+		$lang = $this->getMockBuilder(Language::class)
+			->getMockForAbstractClass();
+
+		// Mock the Document object.
+		$doc = new Document(
+			[
+				'factory' => $this->createMock(FactoryInterface::class),
+			]
+		);
+
+		// Mock WA and some calls used that we are not directly testing.
+		$wa = $this->createMock(WebAssetManager::class);
+		$wa->expects($this->any())->method('__call')->will($this->returnValue($wa));
+		$doc->setWebAssetManager($wa);
+
+		// Inject the mocked document in the mocked factory.
+		$factory::$document = $doc;
+
+		// Mock a template that the app will return from getTemplate().
+		$template = new stdClass;
+		$template->template = 'system';
+		$template->params = new Registry;
+		$template->inheritable = 0;
+		$template->parent = '';
+
+		// Ensure the application can return all our mocked items.
+		$app = $this->createMock(CMSApplication::class);
+		$app->method('__get')
+			->with('input')
+			->willReturn($input);
+		$app->method('getLanguage')
+			->will($this->returnValue($lang));
+		$app->method('getDocument')
+			->will($this->returnValue($factory::$document));
+		$app->method('getTemplate')
+			->will($this->returnValue($template));
+
+		// Finally, set the application into the factory.
+		$reflection = new ReflectionClass($factory);
+		$reflection_property = $reflection->getProperty('application');
+		$reflection_property->setAccessible(true);
+		$reflection_property->setValue($factory, $app);
+	}
 }