found the purpose of the unknown value

common/src/main/scala/net/psforever/packet/game/ZoneInfoMessage.scala

@@ -6,18 +6,22 @@ import scodec.Codec
 import scodec.codecs._
 
 /**
-  * na
+  * Change the "Empire Status" text in the Interstellar Map zone description for the specified zone.<br>
+  * <br>
+  * The `Long` value is applicable to the caverns.
+  * It indicates how long the given cavern will remain locked until at least one of their geowarps will open during normal rotation.
+  * As thus, when a cavern has its status as "inactive," this number will always be a non-zero.
+  * On normal continents, this number is always zero, though a non-zero number will not have any effect anyway.
   * @param zone the zone id
-  * @param empire_status change the "Empire Status" text in the Interstellar Map zone description;
-  *                      `true` displays the queue availability for the avatar's empire;
-  *                      `false` display "Inactive"
-  * @param unk na;
-  *            always 0?
-  * @see `ZonePopulationUpdateMessage`
+  * @param empire_status `true` displays the queue availability for the avatar's empire;
+  *                      `false` displays "Inactive"
+  * @param lock_time how long until the continent naturally unlocks (in ms);
+  *                  only applicable to caverns
+  * @see `ZonePopulationUpdateMessage` for information on population and queues
   */
 final case class ZoneInfoMessage(zone : Int,
                                  empire_status : Boolean,
-                                 unk : Long)
+                                 lock_time : Long)
   extends PlanetSideGamePacket {
   type Packet = ZoneInfoMessage
   def opcode = GamePacketOpcode.ZoneInfoMessage
@@ -28,6 +32,6 @@ object ZoneInfoMessage extends Marshallable[ZoneInfoMessage] {
   implicit val codec : Codec[ZoneInfoMessage] = (
     ("zone" | uint16L) ::
       ("empire_status" | bool) ::
-      ("unk" | uint32L)
+      ("lock_time" | uint32L)
     ).as[ZoneInfoMessage]
 }