Where an American would write 12/1/91 and know this is the 1st day of the 12th month of the year 1991, I would understand this is the 12th day of January 1891.
In Europe 12/1/1991 is the 12th January 1991, in America it would be the 1st December 1991.
Moreover, since you are dealing with history, "91" could mean anything: 1991, 1891, 1791, 1691, ...
The format used by genealogists is 12 Jan 1991, or 1 Dec 1891. Even if you would prefer the format Dec 1, 1891, you should always mention the month at least with its three first characters: Jan, Feb, Mar, Apr, May, Jun, Jul, Aug, Sep, Oct, Nov, Dec, and always write the year in full, with the four digits.
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September, October, November, December
In ancient Rome, BC, they only had ten months, of which the first six had a name: Januarius, Februarius, Martius, Aprilis, Maius, Junius. The next ones only had a number: seven, eight, nine, ten, but in Latin of course: septem, octo, novem, decem.
At one time they decided that they actually needed twelve months, and they named these months after some of their famous emperors: Julius and Augustus. Logically, these named months were placed after the other named months: Januaris, Februarius, Martius, Aprilis, Maius, Junius, Julius, Augustus, but this also meant that the 7th, 8th, 9th and 10th month moved up two places. Therefore the seventh month actually became the ninth, the eight the tenth, etc.
In earlier times, and still early in this century, many people, especially those who had still learned Latin, wrote the abbreviated forms:
- 7ber = September means 7th month, but is actually the 9th
- 8ber = October means 8th month, but is actually the 10th
- 9ber = November means 9th month, but is actually the 11th
- 10ber = december means 10th month, but is actually the 12th
In some cases you may also find Roman numerals used: VIIber , VIIIber, IXber, Xber
The "ber" is usually in superscript: 7ber, 8ber, 9ber, 10ber, or VIIber , VIIIber, IXber, Xber
The ending -ber becomes -bre in French.
[note: as usual, things in Rome were a bit more complicated than described here, but as long as you remember that the last months of the year moved up two places, this article served its purpose]