$wpdb->insert() and Values for Datetime Columns?

I have a custom table with a DATETIME column. I have a function to add records; I would like to add an $expires argument to this function which can hold a unix timestamp noting when the record should expire. I would like to use $wpdb->insert() to populate this column. So far I’ve been unable to find a WordPress convenience function to massage the timestamp into ‘YYYY-MM-DD HH:MM:SS’ format. Is there such a builtin, or should I just use $wpdb->prepare(), or possibly a third option?