If you are using djbdns instead of bind, the zone file is documented at https://cr.yp.to/djbdns/tinydns-data.html

Note the neat djbdns feature where you can set the starting and ending time for every single record. "tinydns dynamically adjusts ttl so that the line's DNS records are not cached for more than a few seconds past the ending time." That makes planned IP address changes a lot less painful!