Meeus is solid on the difference between the mean tropical year and the vernal equinox year, apparently (http://www.hermetic.ch/cal_stud/cassidy/err_trop.htm).
If you're trying to plot the terminator on the earth's surface, you'll have to deal with uncertainty in the length of the day. For instance, leap seconds cannot be predicted much more than six months in advance. Worse, the uncertainty here is quadratic: the earth might speed up or slow down. This is mostly due to massy magma currents, but apparently even damming large bodies of water near the equator makes a difference.
You can use universal earth time instead of absolute time, of course, but that shifts the error to the time of year. But it might be less significant there.
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If you're trying to plot the terminator on the earth's surface, you'll have to deal with uncertainty in the length of the day. For instance, leap seconds cannot be predicted much more than six months in advance. Worse, the uncertainty here is quadratic: the earth might speed up or slow down. This is mostly due to massy magma currents, but apparently even damming large bodies of water near the equator makes a difference.
You can use universal earth time instead of absolute time, of course, but that shifts the error to the time of year. But it might be less significant there.