Add alter hooks for a number of settings.

recurring_events.api.php

+<?php
+
+/**
+ * @file
+ * Custom hooks exposed by the recurring_events module.
+ */
+
+/**
+ * Alter the time options available when creating an event series entity.
+ *
+ * @param array $times
+ *   An array of times in the format h:i a.
+ */
+function hook_recurring_events_times_alter(array &$times = []) {
+  // Events cannot occur at midnight.
+  unset($times['00:00 am']);
+}
+
+/**
+ * Alter the duration options available when creating an event series entity.
+ *
+ * @param array $durations
+ *   An array of durations in seconds.
+ */
+function hook_recurring_events_durations_alter(array &$durations = []) {
+  // Events can last for 2 days.
+  $durations[172800] = t('2 days');
+}
+
+/**
+ * Alter the days options available when creating an event series entity.
+ *
+ * @param array $days
+ *   An array of available days.
+ */
+function hook_recurring_events_days_alter(array &$days = []) {
+  // No events can take place on sundays.
+  unset($days['sunday']);
+}
+
+/**
+ * Alter the month days options available when creating an event series entity.
+ *
+ * @param array $month_days
+ *   An array of available days of the month.
+ */
+function hook_recurring_events_month_days_alter(array &$month_days = []) {
+  // No events can take place on the 17th of a month.
+  unset($month_days[17]);
+}

src/Plugin/Field/FieldWidget/MonthlyRecurringDateWidget.php

@@ -137,7 +137,7 @@ class MonthlyRecurringDateWidget extends WeeklyRecurringDateWidget {
 
     $days[-1] = t('Last');
 
-    // TODO: Add hook ability to modify these days.
+    \Drupal::moduleHandler()->alter('recurring_events_month_days', $days);
 
     return $days;
   }

src/Plugin/Field/FieldWidget/WeeklyRecurringDateWidget.php

@@ -142,7 +142,7 @@ class WeeklyRecurringDateWidget extends DateRangeDefaultWidget {
       $times[$time_option->format('h:i a')] = $time_option->format($format);
     }
 
-    // TODO: Add hook ability to modify these times.
+    \Drupal::moduleHandler()->alter('recurring_events_times', $times);
 
     return $times;
   }
@@ -189,7 +189,7 @@ class WeeklyRecurringDateWidget extends DateRangeDefaultWidget {
       '86400' => t('24 hours'),
     ];
 
-    // TODO: Add hook ability to modify these durations.
+    \Drupal::moduleHandler()->alter('recurring_events_durations', $durations);
 
     return $durations;
   }
@@ -215,7 +215,7 @@ class WeeklyRecurringDateWidget extends DateRangeDefaultWidget {
       'sunday' => t('Sunday'),
     ];
 
-    // TODO: Add hook ability to modify these days.
+    \Drupal::moduleHandler()->alter('recurring_events_days', $days);
 
     return $days;
   }