What the date does establish
Across every year from 1930 to 2035, the noon Sun on 28 September lies between 4.4 and 5.8 degrees of Libra. That is a settled Libra position, in the first decan of the sign, with the equinox behind it by the best part of a week.
It also holds regardless of the hour. Solar motion is about a degree a day, so even the extreme case of midnight against midnight moves the figure by less than one degree and cannot reach a boundary. The Sun is the one thing a date can pin down, and it does so comfortably here.
For the finer division, this date falls in the first decan of Libra, the band from zero to ten degrees, in every year of the range. The Chaldean order rules that band with the Moon under Venus as ruler of the sign, and the triplicity order names it the Libra decan and keeps Venus in both places.
The Moon, which the date halves rather than settles
The Moon covers roughly thirteen degrees of the ecliptic each day and finishes a sign in a little over two days. Whether it changes sign during any particular 28 September depends on the year, and when it does, the hour of birth decides the answer.
Traditional practice puts a great deal on the Moon, reading it for temperament, habit and response rather than for stated identity. A page that gives a sun sign and calls the job done has skipped the placement that many practitioners would consult first. The best a date alone can do is narrow it to two.
The Ascendant, which the date does not touch
The rising sign is whichever part of the ecliptic was climbing over the eastern horizon at the moment of birth. The Earth turns once in a day, so every one of the twelve signs rises in the course of any date, roughly two hours apiece. A birthday excludes none of them.
The Ascendant also anchors the houses, the twelve divisions that connect placements to areas of life. Without a birth time the houses cannot be built, and a chart drawn without them is a list of positions rather than a description. This is the specific reason birth certificates matter to astrologers.
The Ascendant is also the most sensitive point in a chart to an error in the recorded time. Four minutes of clock error moves it a degree, and a difference of an hour or two moves it a whole sign, so a birth time remembered rather than recorded should be treated as approximate and the resulting houses as provisional.
Working with what you have
If the birth hour is unavailable, the standard approach is to calculate for noon and to mark the fast placements as provisional. On this date the Sun survives that treatment untouched. The Moon should be recorded as one of two options, and the Ascendant left blank rather than guessed.
The widget above does this properly. Supply the year and birthplace, add the hour if you have it, and it returns the exact solar longitude together with how far the birth stood from the nearest sign boundary. On 28 September that distance is measured in days, which is the plain reason no cusp claim applies.
Questions people ask about this
What sign is 28 September?
Libra, without exception in the years calculated. The Sun sits between 4.4 and 5.8 degrees of the sign at noon, roughly six days after the equinox that opens it.
Can a birthday tell me my Moon sign?
Only partly. The Moon changes sign every couple of days, so a date usually leaves two possibilities and the birth hour decides which applies.
How do I find my rising sign?
You need the birth time and place as well as the date, because the Ascendant moves through all twelve signs in a day. Without the hour, none of the twelve can be ruled out from the date alone.
Is the sun sign the most important placement?
Not in traditional practice. It is the most widely quoted because it is the only one a date reveals, which is a fact about publishing rather than about astrology.
Which decan is 28 September?
The first decan of Libra, running from zero to ten degrees, in every year of the range. The Chaldean scheme assigns that band to the Moon.
Does my birthplace change my sun sign here?
No. Time zones shift the local clock by hours, and the Sun would need several days of motion to leave Libra from this position.