How to generate a random seed at a 24 hour interval

When generating the seed, store it somewhere (file / playerprefs / ...), along the date the seed has been created.

When you must use the seed, check if it has "expired" (ie : Time.time - saveTime > 3600 * 24)

EDIT : If all the players must play on a same map the same day, you will have to get the date from somewhere (ins$$anonymous$$d of Time.time). $$anonymous$$aybe, you can have a solution using System.DateTime.UtcNow

https://msdn.microsoft.com/en-us/library/system.datetime.utcnow(v=vs.110).aspx

Depends how much you're worried about users changing the date on their device.

  using UnityEngine;
  using System.Collections;
  using System;
  
  public class UTC: $$anonymous$$onoBehaviour 
  {
      void Start()
      {
          print (DateTime.UtcNow);
      }
  }

Should show the current device date/time in UTC but it's open to exploits.

 var url = "http://mytimeserver/gettime";
 var dateTime : System.DateTime;
  
 function Start () {
     var www : WWW = new WWW (url);
  
     // Wait for download to complete
     yield www;
    
     dateTime = DateTime.Parse(www.text);
 }

Should work (although I googled that!) if you have a reliable time server you can request from.

Just poll the server in a Start() and have the game keep track of time from there.

$$anonymous$$inda off topic but I'd also change the tick over time for the maps, if you always start at the same time some time zones will have a natural advantage.