Actually, the days were still getting shorter but since December 16th the sunsets have been getting later by a few seconds each day because Earth's rotation isn't a perfect 24 hours. It's hard to explain but the actual time for Earth to complete a revolution, relative to stellar positioning, is 23 hr 59 m 54 seconds but since Earth is orbiting the sun it is deceptive because the actual time it takes Earth to complete a revolution, relative to our own SUN, is 24 hr 00 m 00.6 seconds - on average. It actually varies because Earth actually doesn't travel through space at a constant speed. It's a brain bender.