The degrees and the days
A sign holds three faces of ten degrees, thirty six in total around the ecliptic. This one begins once the Sun has passed ten degrees of Pisces and ends at twenty.
The Sun is generally in it from about 1 March to about 11 March. Treat that as approximate. Since the tropical year is not a whole number of days, every boundary in the zodiac shifts by up to a day and a half from year to year, and a birth time can move it again. Leap years pull the dates in one direction and the years around them push back, which is why published ranges rarely quite agree with each other.
The middle of the range is safe; it is the first and last day or so that need care.
Jupiter and the Moon
The Chaldean order proceeds through Mars, Sun, Venus, Mercury, Moon, Saturn and Jupiter in a loop that starts at zero degrees of Aries and never resets. The thirty fifth face lands on Jupiter, which is also the traditional ruler of Pisces, so the sign's older lord returns here by a route that has nothing to do with the sign.
The triplicity scheme allots a sign's faces to the signs of its element in order. Pisces, Cancer and Scorpio make up the water triplicity, and Pisces takes them as Pisces, Cancer, Scorpio, so this middle face belongs to Cancer and to the Moon.
Jupiter and the Moon differ, but not in the way that Saturn and the Sun differ. Both are read as receptive rather than restrictive, so the descriptions produced by either scheme overlap more than usual. That does not make the distinction unimportant, and a source quoting one ruler without naming its method is still leaving out half the picture.
What is read into the combination
The Cancer colouring is read as Pisces with an attachment. Where the sign on its own is described as diffuse, this face is described as directed at particular people: a household, a family, a small circle it will inconvenience itself for. Traditional writing links it to memory and to the past being carried forward, and to a protectiveness that is exercised quietly rather than declared.
The Chaldean Jupiter is read as widening that circle. Older texts associate Jupiter with generosity extended past the people one already knows, and with an interest in meaning rather than in mood alone, so here it is usually interpreted as sympathy that turns into a stated principle instead of stopping at feeling. The same texts warn that Jupiter in water can promise more than the person can actually carry.
None of this belongs to the Sun alone. Any placement occupies a decan, and the Ascendant and the Moon are read this way constantly. Decans reach us from Egyptian star lists that predate the twelve signs in their current form, which is why the technique retains its standing, but it is a second-order refinement rather than anything a reading rests on.
Working out the degree
The boundaries here are degrees, not dates. A birthday on 1 March or 11 March may fall on either side depending on the year and the hour, because the Sun covers roughly a degree a day.
The check above computes the Sun's actual longitude for your birth date and gives the degree with the ruler each scheme assigns. It also names the neighbouring face, so a close result is easy to read. A published date range cannot do any of that.
Questions people ask about this
What dates are the second decan of Pisces?
Approximately 1 to 11 March. Both edges move by up to a day and a half between years, so anyone born at either end should have the solar degree worked out rather than rely on a range.
Is the ruler here Jupiter or the Moon?
Jupiter under the Chaldean order and the Moon through the triplicity scheme, which gives the face to Cancer. Their readings sit closer together than most disagreements between the schemes, but they are not identical.
Why does Jupiter turn up in a Pisces decan by chance?
The Chaldean loop runs continuously from the start of Aries and takes no notice of sign boundaries, so a sign's own traditional ruler occasionally reappears in one of its faces. Here that happens with Jupiter.
Does the Cancer face make someone more emotional?
The tradition reads it as more attached to specific people rather than more emotional in general. That is a statement about where feeling is directed, and the rest of the chart decides how it shows.
Can I apply this to my Moon?
Yes, and a water decan on a natal Moon is one of the places practitioners give the technique most weight. Every placement, including the Ascendant, sits in one of the thirty six decans.
Should a decan change how I read my chart?
Only at the margins. It is a documented part of the tradition with a long history behind it, but it adds shading to a placement rather than altering the sign, house and aspects that carry the reading.