You can only get this achievement during the 1 minute following midnight. If you don't want to wait until midnight, but you still want to get this achievement online, then from the dashboard, go to the System Settings menu, select Console Settings, select Clock, select Time Zone and choose a time zone where it will be midnight within the next hour.
Start Halo 4 and load up the second campaign mission (Requiem) on any difficulty. Run through the valley until you reach the Warthogs, get in one and drive round the hill until you reach the tunnel that goes into the side of the hill and position yourself at the bottom of the ramp. This should take about 5 minutes. Press the Guide (Xbox) button and check the time. You still have to wait until your Xbox thinks it is 0:00 (or 12:00 AM), but at least you will not have to wait more than an hour. As soon as the clock changes to 0:00, press the Back button (or B) and drive at full speed up the ramp. The achievement will unlock as soon as you land. You can then exit the game and set your Time Zone back.
Of course if you don't mind this achievement showing as won offline with no date, you can set the time on your Xbox to 23:50 (or 11:50 PM) and the follow the instructions in the above paragraph. You can only change the clock if you are not connected to Xbox LIVE, here are the 3 ways you can do this.
1. If you have the original Xbox, you can unplug the network cable and disconnect the wireless adapter if you have one.
2. If you have the slim Xbox you cannot disconnect the wireless adapter so you will either have to unplug your wireless router from your cable connection or wireless gateway from your phone line or wireless access point from your network. Alternatively from System Settings - Network Settings select your wireless network and press Y to Disconnect (thanks to Tomblyboo117
for this tip). You will need you WPA or WEP pass key to reconnect.
3. Add a password to your Xbox account and set your account to auto logon. That way when you turn on your 360 you will always be logged on offline.
I used to use method 3. Now I have a slim and I have it wired to the network but have not entered my WEP passkey, so it can't connect to my wireless network when I unplug the network cable.