删除函数FROM_UNIXTIME中的单引号，因为它将使它成为一个值.例如

remove single quotes aroung the function FROM_UNIXTIME because it will make it a value. eg

$sql = "INSERT INTO `calendar` (`date`, `title`, `event`) VALUES (FROM_UNIXTIME(".$datestr."), '".mysql_real_escape_string($_POST['title'])."', '".mysql_real_escape_string($_POST['desc'])."')";

作为附带说明，您的查询容易受SQL Injection的攻击，请参阅下面的文章以了解如何保护它免于攻击

As a sidenote, your query is vulnerable with SQL Injection, please see the article below to learn how to protect from it

• 如何防止PHP中的SQL注入?