#1847 Fix datepicker to respect the searchDate offsets

I have some misgivings about this since I suspect the rationale behind the search offsets was primarily about the clunky UI.

Having said that I think the datepicker UI is a bit clunky on this front too as it's not obvious you can choose earlier
dates. This does at least restore established behaviour.

#1847

CRM/Core/Form.php

@@ -359,6 +359,8 @@ class CRM_Core_Form extends HTML_QuickForm_Page {
    *
    * @return HTML_QuickForm_Element
    *   Could be an error object
+   *
+   * @throws \CRM_Core_Exception
    */
   public function &add(
     $type, $name, $label = '',
@@ -385,16 +387,34 @@ class CRM_Core_Form extends HTML_QuickForm_Page {
       unset($attributes['multiple']);
       $extra = NULL;
     }
-    // @see http://wiki.civicrm.org/confluence/display/CRMDOC/crmDatepicker
-    if ($type == 'datepicker') {
-      $attributes = ($attributes ? $attributes : []);
+
+    // @see https://docs.civicrm.org/dev/en/latest/framework/ui/#date-picker
+    if ($type === 'datepicker') {
+      $attributes = $attributes ?: [];
+      if (!empty($attributes['format'])) {
+        $dateAttributes = CRM_Core_SelectValues::date($attributes['format'], NULL, NULL, NULL, 'Input');
+        if (empty($extra['minDate']) && !empty($dateAttributes['minYear'])) {
+          $extra['minDate'] = $dateAttributes['minYear'] . '-01-01';
+        }
+        if (empty($extra['maxDate']) && !empty($dateAttributes['minYear'])) {
+          $extra['maxDate'] = $dateAttributes['maxYear'] . '-12-31';
+        }
+      }
+      // Support minDate/maxDate properties
+      if (isset($extra['minDate'])) {
+        $extra['minDate'] = date('Y-m-d', strtotime($extra['minDate']));
+      }
+      if (isset($extra['maxDate'])) {
+        $extra['maxDate'] = date('Y-m-d', strtotime($extra['maxDate']));
+      }
+
       $attributes['data-crm-datepicker'] = json_encode((array) $extra);
       if (!empty($attributes['aria-label']) || $label) {
         $attributes['aria-label'] = CRM_Utils_Array::value('aria-label', $attributes, $label);
       }
       $type = "text";
     }
-    if ($type == 'select' && is_array($extra)) {
+    if ($type === 'select' && is_array($extra)) {
       // Normalize this property
       if (!empty($extra['multiple'])) {
         $extra['multiple'] = 'multiple';