Ticket #36259: 36259.2.patch

diff --git a/src/wp-admin/options.php b/src/wp-admin/options.php
index 8348b24c8e941..3a8ef92ae8850 100644
--- a/src/wp-admin/options.php
+++ b/src/wp-admin/options.php
@@ -264,18 +264,24 @@
                $options = $allowed_options[ $option_page ];
        }

+       $using_locales_default_date_format = false;
+       $using_locales_default_time_format = false;
+
        if ( 'general' === $option_page ) {
                // Handle custom date/time formats.
                if ( ! empty( $_POST['date_format'] ) && isset( $_POST['date_format_custom'] )
                        && '\c\u\s\t\o\m' === wp_unslash( $_POST['date_format'] )
                ) {
                        $_POST['date_format'] = $_POST['date_format_custom'];
+               } elseif ( __( 'F j, Y' ) === $_POST['date_format'] ) {
+                       $using_locales_default_date_format = true;
                }

                if ( ! empty( $_POST['time_format'] ) && isset( $_POST['time_format_custom'] )
                        && '\c\u\s\t\o\m' === wp_unslash( $_POST['time_format'] )
                ) {
                        $_POST['time_format'] = $_POST['time_format_custom'];
+               } elseif ( __( 'g:i a' ) === $_POST['time_format'] ) {
+                       $using_locales_default_time_format = true;
                }

                // Map UTC+- timezones to gmt_offsets and set timezone_string to empty.
@@ -353,6 +359,16 @@
                $user_language_new = get_user_locale();
                if ( $user_language_old !== $user_language_new ) {
                        load_default_textdomain( $user_language_new );
+
+                       // Update formats only if locale defaults were previously used.
+                       if ( $using_locales_default_date_format ) {
+                               update_option( 'date_format', __( 'F j, Y' ) );
+                       }
+
+                       if ( $using_locales_default_time_format ) {
+                               update_option( 'time_format', __( 'g:i a' ) );
+                       }
                }
        } else {
                add_settings_error( 'general', 'settings_updated', __( 'Settings save failed.' ), 'error' );
diff --git a/tests/phpunit/tests/i18n/test-switching-locale-updates-date-time-formats.php b/tests/phpunit/tests/i18n/test-switching-locale-updates-date-time-formats.php
new file mode 100644
index 0000000000000..92f54494f9451
--- /dev/null
+++ b/tests/phpunit/tests/i18n/test-switching-locale-updates-date-time-formats.php
@@ -0,0 +1,120 @@
+<?php
+/**
+ * @ticket 36259
+ * @group i18n
+ */
+
+class Tests_I18n_Switching_Locale_Updates_Date_Time_Formats extends WP_UnitTestCase {
+
+       private $orig_locale;
+       private $orig_date_format;
+       private $orig_time_format;
+
+       public function set_up() {
+               parent::set_up();
+
+               $this->orig_locale      = get_locale();
+               $this->orig_date_format = get_option( 'date_format' );
+               $this->orig_time_format = get_option( 'time_format' );
+
+               switch_to_locale( 'en_US' );
+               update_option( 'date_format', 'F j, Y' );
+               update_option( 'time_format', 'g:i a' );
+       }
+
+       public function tear_down() {
+               switch_to_locale( $this->orig_locale );
+               update_option( 'date_format', $this->orig_date_format );
+               update_option( 'time_format', $this->orig_time_format );
+
+               parent::tear_down();
+       }
+
+       public function test_switching_locale_updates_formats_when_defaults_used() {
+               global $l10n;
+
+               $translations = new MO();
+               $translations->add_entry(
+                       new Translation_Entry(
+                               array(
+                                       'singular'     => 'F j, Y',
+                                       'translations' => array( 'j F Y' ),
+                               )
+                       )
+               );
+               $translations->add_entry(
+                       new Translation_Entry(
+                               array(
+                                       'singular'     => 'g:i a',
+                                       'translations' => array( 'H:i' ),
+                               )
+                       )
+               );
+
+               $l10n['default'] = &$translations;
+
+               switch_to_locale( 'en_GB' );
+
+               update_option( 'date_format', __( 'F j, Y' ) );
+               update_option( 'time_format', __( 'g:i a' ) );
+
+               $this->assertSame( 'j F Y', get_option( 'date_format' ) );
+               $this->assertSame( 'H:i', get_option( 'time_format' ) );
+       }
+}