They didn't because this type of numeracy system was created by the mysterious Etruscans and not much is known in history about them.
it was used when the Romans were still around that's why its called Roman Numerals
In today's modern notation of Roman numerals it is MCMXCV but the ancient Romans probably have wrote it out quite differently
It will be as the ancient Romans would have notated it MMLXXXXVIIII = 2099
In Roman numerals it is the year MCMLXIV. In Arabic numerals it is the year 1964.
The year 1450 is written in roman numerals as MCDL.
Nowadays the equivalent of 1961 in Roman numerals are MCMLXI but the ancient Romans would have notated them quite differently
In today's terms it is represented by the numerals MCMXCVIII but the Romans themselves would probably have written it as IIMM (-2+2000 = 1998).
In today's notation of Roman numerals 1910 is MCMX but the ancient Romans would have notated it quite differently.
It is not equivalent to any year because they are an invalid arrangement of Roman numerals
MCMXXI represents the year 1921 in Roman numerals.
The year 1958 is written as MCMLVIII in Roman numerals.
The number (or year) 2009 in Roman numerals is MMIX