1888 is equal to MDCCCLXXXVIII in its additional notation.
But 1999 is equal to MDCCCCLXXXXVIIII in its additional notation.
The longest Roman numeral with a value under 1000 would be DCCCLXXXVIII (888)
1888 MDCCCLXXXVIII
I believe it is 88 = LXXXVIII
The longest Roman numeral under 1000 is CMXCIX, which represents the number 999. This is the longest because it combines the largest Roman numeral symbols in descending order from left to right: CM for 900, XC for 90, and IX for 9. Roman numerals are additive, so the longest numeral is typically one that requires the most symbols to represent a number.
Under today's rules now governing the Roman numeral system the given numeral is equivalent to 289
Under today's rules now governing the Roman numeral system the Roman numeral for 54 is LIV but the ancient Romans would have notated it as LIIII
Under today's rules now governing the Roman numeral system 946 as a Roman numeral is CMXLVI but the ancient Romans would have notated it quite differently
Under the rules now governing the Roman numeral system 1949 is deemed to be MCMXLIX as a Roman numeral but the ancient Romans would have notated it as MDCCCCXXXXVIIII
MCMLX under today's rules governing the Roman numeral system they represent the year 1960
Under today's rules now governing the Roman numeral system CXCIX represents 199
Under the rules now governing the Roman numeral system 1963 as a Roman numeral is MCMLXIII but the ancient Romans would have notated it quite differently
Under today's rules now governing the Roman numeral system MCMLI is equivalent to 1951