Count down in the MOTD field.

CountdownMod.java

package com.comphenix.example;

import java.text.DateFormat;
import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.TimeZone;

import org.apache.commons.lang.time.DateUtils;
import org.bukkit.event.EventHandler;
import org.bukkit.event.Listener;
import org.bukkit.event.server.ServerListPingEvent;
import org.bukkit.plugin.PluginManager;
import org.bukkit.plugin.java.JavaPlugin;

public class CountdownMod extends JavaPlugin implements Listener {
    // The time the game starts
    private Date launchTime;
    
    @Override
    public void onEnable() {
        PluginManager manager = getServer().getPluginManager();
        manager.registerEvents(this, this);
        
        // Start the game in one hour.
        launchTime = DateUtils.addHours(new Date(), 1);
    }
    
    @EventHandler
    public void onServerListPing(ServerListPingEvent event) {
        Date currentTime = new Date();
        DateFormat formatter = SimpleDateFormat.getTimeInstance();
        formatter.setTimeZone(TimeZone.getTimeZone("GMT"));
        
        // See if the game has event started
        if (launchTime == null) {
            event.setMotd("Game has not started.");
        } else {
            // Note: Doesn't take into account leap seconds, ect. 
            long milliDelta = launchTime.getTime() - currentTime.getTime();
            Date dateDelta = new Date(Math.abs(milliDelta));
            
            if (milliDelta > 0) {
                event.setMotd("Game will start in " + formatter.format(dateDelta));
            } else {
                // You can also print the duration of the game
                //event.setMotd("Game has been running for " + formatter.format(dateDelta));
                event.setMotd("Game has started.");
            }
        }
    }
}