The time in-game is the same as the time on your computer. So, simply load and play a mission at midnight to unlock.
If you can't wait, you can change the time of day by altering your clock. Simplest way is to change the time zone by closing the game, right-clicking your computer's clock and Adjust date/time, and set your time to a couple of minutes to midnight, and open your game again.
Simply play until the clock strikes midnight!
Edit: To clarify, you need to be playing the game at midnight, hence why we're changing the clock to just before midnight.