I receive the same error message.
1. In sermon details, I do set a date using the calendar.
2. However, after receiving the noted error message, when you visit “Sermons>Sermons” the sermons are listed, and in the “Date Preached” column it shows the following error:
Warning: date() expects parameter 2 to be long, string given in /home/removed/public_html/wp-content/plugins/sermon-manager-for-wordpress/includes/template-tags.php on line 224
So there appears to be a problem with the date.