Why isn't the solstice New Years Day?

Adding to what NoNukes has said, the rotation of the Earth slows by a few milliseconds per day per century, and has gained that 0.002 sec since the second was defined. This continuous slowing has to be taken into account in determining the local time for astronomical events in the past, as it adds up to a matter of hours BC. If civilisation survives, and the second is not redefined, our distant descendants will need a leap second every day!

I'm not sure I understand the argument precisely.But... if what you say actually does happen... instead of "redefining the second" to correct the anomaly in the rotation of the Earth... couldn't we simply redefine "how long a day is" to be 24 hours (and 1 second)?That way, the time base itself is still the same... it's just that the day has gotten longer... which is actually what's happened physically as well (isn't it?).

In round figures, we currently need a leap second every 18 months. At current rates of slowing, in 100 years there will be one needed every 9 months, in 200 every 6 months, 1000 years about every 2 months, 10,000 years every week, 50,000 to 100,000 years it will reach every day. Our vastly superior descendants will work out an answer. (Or our post-civilisation cave-dwellers won't care!)