The exact stretch of Pisces
A sign is thirty degrees wide, and the Sun crosses it in roughly thirty days. Saying Pisces tells you which thirty degree band the Sun was in. Saying twenty seven degrees tells you where inside it, which is the part almost no birthday page bothers to publish.
On this date the noon position has stayed between about 27.0 and 28.3 degrees for every year in the sampled century. The birthday sits two or three degrees short of the end of the zodiac, close enough to the boundary to be interesting and far enough from it that no year, hour or birthplace turns the answer into Aries.
The difference is practical rather than decorative. A longitude tells you which planets your Sun aspects and where it falls relative to a house cusp, while a sign name tells you only which thirtieth of the circle it occupies. Two people a fortnight apart in Pisces share the sign and share almost none of the aspect picture, and the degree is what shows that.
Why the position holds so still
The tropical year runs about 365.2422 days, so an ordinary calendar year finishes a fraction early and the Sun arrives about a quarter of a degree further on at the same clock time the next year. Four of those fractions add up to nearly a full day, and the leap day takes it back.
The result is a small sawtooth rather than a drift. Across a century the whole spread on this date is a degree and a third. Your own year decides where in that sawtooth you fall, and the widget above works it out from your year, hour and birthplace rather than from an average.
What Mars is doing in a water sign
The Chaldean order hands the third decan of Pisces to Mars. That is the older subdivision of the zodiac, thirty six bands of ten degrees each with a planet attached, and it predates most of the aspect work that modern readers use daily.
Traditional sources read the combination as feeling that finally acts, since Jupiter and in modern practice Neptune govern the sign while Mars governs only this last third of it. Whether any of that describes a person is a question the tradition cannot answer and neither can a date. The band is measurable, the character attached to it is inherited interpretation, and it is worth keeping the two apart when you read them.
Everything the date does not fix
The Moon moves about thirteen degrees a day, so on any given 18 March it can be in either of two signs and occasionally a third. Without an hour attached, the date narrows it to a pair at best.
The rising sign is not touched by the date at all. All twelve signs come up over the horizon in the course of a day, so the Ascendant is decided by the clock time and the latitude of the birthplace, not by the calendar square. That is why the sun sign is the least individual part of a chart despite being the only part most people are ever told.
Mercury and Venus do get narrowed. Both stay near the Sun, so on this date Mercury is generally within a sign of Pisces and Venus within two, which is a real constraint even though the exact degrees still need a year.
None of this makes the sun sign wrong. It makes it partial, which is a different complaint, and one that a birthday page is in a good position to be honest about rather than to paper over.
Questions people ask about this
Is 18 March always Pisces?
Yes. The Sun does not reach the start of Aries until around the twentieth of the month, so this date sits clear of the boundary in every year sampled. No combination of birth hour and time zone changes it.
Which decan is an 18 March birthday in?
The third decan of Pisces, covering twenty to thirty degrees. In the Chaldean scheme that band is assigned to Mars, so a page listing Neptune or Pluto for it is using a different and more modern allocation.
How close is this date to Aries?
About two degrees, which the Sun covers in roughly two days. Close enough that the next boundary is in sight, far enough that the sign is not in any doubt.
Does a late degree make the sign weaker?
Not technically. A Sun at twenty eight degrees of Pisces is as much in Pisces as one at three. The degree matters for aspects and for which house the Sun lands in, not for whether the sign counts.
Why do two sources give different degrees for my birthday?
Usually because one of them is quoting a noon position and the other has used your actual birth time and place. A single day moves the Sun a full degree, so the hour genuinely shifts the answer.
What do I still need to work out a whole chart?
The birth time and the birthplace. Those two fix the Ascendant, the house cusps and the Moon to a degree rather than to a pair of possible signs.