A boundary inside the sign
Decans divide each sign into three bands of ten degrees. The first decan of Aries runs from zero to ten and belongs to Mars in the Chaldean order, and the second runs from ten to twenty and belongs to the Sun.
Most years put this birthday just under the ten degree mark, in the Mars band. Years at the top of the leap cycle can push a noon position past it. Since the Sun covers about a degree a day, a birth in the evening rather than the morning can also matter here in a way it does not for a mid sign date.
What actually changes at the line
Crossing a decan boundary does not change the sign, the element or the sign ruler. Aries stays Aries, stays fire, stays ruled by Mars. What changes is the planet attached to the ten degree band, and with it the secondary reading the tradition assigns.
The older texts describe the Mars band as the sign undiluted, since the decan repeats the sign ruler, and the Sun band as the same material with an interest in visibility and standing laid over it. Those are inherited characterisations of the divisions rather than statements about people born in them, and they are worth reading with that label attached.
The Chaldean sequence is what produces both assignments. It runs Saturn, Jupiter, Mars, Sun, Venus, Mercury, Moon and is applied without a break to the thirty six bands starting from zero degrees of Aries, so the planets fall where they fall by position in the run rather than by any argument about the sign.
Why the year decides
The whole spread across a century is only about a degree and a third, but on this date that band happens to contain a division, so the small variation has a visible consequence.
The mechanism behind the variation is ordinary. A common year is roughly a quarter of a day short of the Sun's circuit, so each year the noon position gains about a quarter of a degree, and the leap day gives back what four years accumulate. Your own year fixes your place in that cycle, and the widget above computes the exact degree from your year, hour and birthplace rather than leaving it to an average.
The birth hour matters here for the same reason. The Sun moves close to a degree a day, so a birth near midnight sits about half a degree away from the noon figure, in one direction or the other depending on the time zone. On a date sitting astride a division, half a degree is enough to change the answer.
The sign is not in question
It is worth separating the two kinds of boundary clearly. A decan division is ten degrees wide inside a sign and can genuinely be uncertain on a date like this. A sign boundary is thirty degrees away in either direction here, which is about a month of solar motion, so no birth data moves this date out of Aries.
The rest of the chart is untouched by the date. The Moon covers thirteen degrees a day and can be in either of two signs, sometimes a third. The Ascendant runs through all twelve in a day and depends on the birth hour and the latitude of the birthplace. Both need a time, and both carry far more of what separates one person's chart from another's than the sun sign does.
Questions people ask about this
Which decan is a 30 March birthday?
Usually the first decan of Aries, ruled by Mars, but the position can reach the second in some years. Since the degree runs from about nine to just past ten, this is a date where checking your own chart matters.
Is the sign ever in doubt?
No. The nearest sign boundary is roughly three weeks of solar motion away, so Aries holds in every year, hour and time zone.
What is the second decan of Aries ruled by?
The Sun, in the Chaldean allocation. The sequence runs through the thirty six bands in the order used for the planetary hours, and the Sun falls fourth from the start of Aries.
How do I find out which side I am on?
Give your year, birth hour and birthplace to the widget above. It computes the exact longitude and reports how far it sits from the nearest division.
Does a birth near a decan line mean a blend?
Not in any technical sense. The Sun sits at one longitude and that longitude is inside one band. Some traditional writers treat placements near a division as worth double checking, which is a caution about data rather than a mixed condition.
Do modern sources agree on the decan rulers?
Not always. Alongside the Chaldean order there are schemes that assign the bands differently, and Egyptian decans are a separate system altogether. Compare readings only within one scheme.