The earth's orbit is ecliptic and asymmetrical. Science prefers neat divisions and imposes symmetry.
This Vernal Equinox is set by the calendar for March 20th. But nature is going to put the sun in the spot crossing the plane of the celestial equator on March 18th.
There are a lot of these differences in nature, planets, star fields that varies from man's desire for even divisions. We have split the Zodiacal constellations into 30 degree increments, despite the fact that some constellations are quite a bit bigger and their star fields occupy much more than 30 degrees (Pisces, Virgo and Aquarius for example) and some are much less than 30 degrees (Aries, Gemini and Cancer for example).
The incongruities between the mathematical order we prefer in contrast to the "chaos" of nature means something if Genesis 1:14 is to be trusted. Despite mankind's desire to make everything fit neatly, God has decreed events to happen on His time-frame.
Star fields overlap. We think we know when one age ends and another begins because of the 30 degrees allocated to each of the twelve constellations. But in the heavens above, Leo invades Cancer, and Pisces invades Aquarius. There is no neat division allowing anyone to say with certainty when God believes one age ends and another has begun. God may have one age begin before another ends for His reasons. Very rarely does man see this.
So you are left to decide if the coming Equinox is March 20th (based on neat divisions of the year) or March 18th (based on the movement of the sun overhead). And also to decide when Pisces ends and Aquarius begins--or if Aquarius has already begun. Many things begin and the world barely notices they are underway. Heaven testifies and men fail to notice.