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1999=MCMXCIX. Is MIM acceptable? See below.
In Roman numerals, I=1, V=5, X=10, L=50, C=100, D=500, M=1000. I=1, II=2, III=3, IV=4, V=5, VI=6, VII=7, VIII=8, IX=9, X=10, XI=11, XII=12, etc. Here we see (for IV=4) that I, before V, is -1, not +1. Also IX=9, XC=90, CD=400, etc. A lesser number before a greater number is negative.
Bigger numbers were sometimes made with a bar over the letter, to indicate "multiply by 1000" (see the second addendum, below). Two examples:
Here I originally spelled out a bunch of rules. This was because some people seem to spend their lives looking for loop holes. They are going to try weird Roman numerals like IXV or IXIX or VVI or IIX. And then they are going to send me email about it. Well, Roman numerals are written more simply and straightforward than that. But it only complicates the situation to make up a bunch of rules about it.
I read, many years ago, that IC (as well as ID and IM and XM and things like that) was illegal. Apparently such numbers were sometimes used in ancient times. I don't know if they were commonly used. So, from what I have gathered, 1999 is MCMXCIX. Is MIM acceptable? Maybe, maybe not.
There is no Roman numeral zero (see below). IIII (IV in Roman numerals) is traditional on clocks; who knows why? See the first addendum, below.
How did they survive without a zero? How could they tell when all of their money was gone? That should be obvious. Their pockets were empty. They had words for that kind of situation: "My pockets are empty" (translate to Latin). This concept is obviously related to numbers (no coins, when one coin was a slightly better situation). But, you don't need an actual number zero, when it is not part of your counting system. They obviously had words for "none" or "nothing," but there was no Roman numeral for zero. See Discovering Zero.
Similarly, in most cases, you can do without negative numbers (nowadays). You need them to describe the situation in which you owe money (or other things), or when you measure temperatures in Montana. Otherwise, you can ignore negative numbers, just as the Romans had no use for a number zero.
Addendum #1:
I see, in Conway and Guy's The Book of Numbers, that the subtraction rule (IV for 4, etc.) was rarely used until medieval times. If that is true, then there were probably some very long Roman numbers: MDCCCCLXXXXVIIII for 1999.
And I have received several emails informing me that the reason that clocks often have IIII on them (instead of IV) is to make the clock face look more balanced. It doesn't look any more balanced to me.
Addendum #2:
It would seem that there was a second method of writing large numbers, both in Roman times and in the middle ages. The symbols varied considerably, but this is essentially what they looked like:
(|)=1000
|))=5000
((|))=10,000
|)))=50,000
(((|)))=100,000
etc.
Sometimes these became C's, I's, and backward C's.