Why the usual answer is so thin
A sun sign is a thirty degree slice of the ecliptic. Saying someone is Pisces places their Sun somewhere inside a band a full month wide, and every person born in that month gets the identical description.
The position inside the band is a different matter. On 7 March the Sun sits between about 16.0 and 17.4 degrees of Pisces, measured at noon across the years from 1930 to 2035. That is a narrow window, and it is a fact about the date rather than a claim about anyone.
None of that makes the sign wrong. It makes it coarse. A description written for thirty degrees of the ecliptic has to fit everyone inside those thirty degrees, which is why the wording in most places is vague enough to fit almost anyone at all.
Why the figure hardly moves
The Sun advances just under a degree a day, so a shift of a whole degree means a shift of about a day in the calendar. The calendar and the orbit are held together by the leap day: four ordinary years let the date slip roughly eighteen hours late, and February the twenty ninth pulls it back.
The result is that a 7 March birth in 1934 and one in 2034 land within a degree and a half of the same point in the zodiac. Your birth year moves you within that window and nothing moves you out of it.
Seventeen degrees means the middle decan
Pisces splits into three faces of ten degrees. This one falls in the second, between ten and twenty, with a comfortable margin on either side, so the year and the hour cannot push it into a neighbouring face.
The Chaldean sequence assigns that face to Jupiter, and the triplicity scheme assigns it to Cancer, whose ruler is the Moon. Tradition reads the pairing as broad and absorbing rather than pointed. Whether that carries any weight is a judgement, and it is marked here as interpretation rather than measurement.
The two schemes disagree about most of the thirty six faces, and popular articles rarely say which one they are using. If a decan ruler you have read elsewhere does not match the one given here, that is usually the reason rather than an error on either side.
The parts the date cannot tell you
The Moon crosses a sign in a little over two days, so a birth on this date can put it in one of two signs, and near a change it can be either side by a matter of hours. The Ascendant is faster still and can be any of the twelve, set by the local clock time and the latitude of the birthplace.
Which is to say the sun sign is the least individual placement you own, not the most. The widget above works out your own degree from your year and location and tells you how far your Sun stood from the nearest boundary.
It is also worth knowing the Sun's distance from the nearest boundary even when the sign is obvious. Positions near an edge behave differently in some techniques, and confirming that yours is nowhere near one closes a question you might otherwise carry around unanswered.
Questions people ask about this
Is 7 March always Pisces?
Yes. The date sits far from both edges of the sign, so the answer holds for every year, every hour and every time zone.
How many degrees into Pisces is 7 March?
About sixteen to seventeen and a half at noon, depending on the year. The widget above gives a single figure once it knows your birth year and place.
Is this birthday on a cusp?
No. A cusp is what people call a date near a sign boundary, and this date is roughly a fortnight from the nearest one. Nothing here is undecided.
What is the decan for 7 March?
The second decan of Pisces. Chaldean rulership puts Jupiter on it, while the triplicity scheme attaches it to Cancer and therefore the Moon.
Does my exact degree change how the sign is read?
It does not change the sign, but it decides which decan applies and it matters for aspects to other planets. A Sun at seventeen degrees contacts different placements than one at two.
What should I look up next if I already know I am Pisces?
The Moon sign and the rising sign, in that order. Both need your birth time, and both vary far more between two people born on the same day than the Sun does.