When i do:

//echo -a $calc(10008162643314973000 - 10008162643314972967)

and got 0, even thought the numbers are obviously different. I thought at first that mIRC was having trouble with large numbers, so i tried:

//echo -a $calc(100081626433149730123123 - 10008162643314972967)

and got 100071618270506410000000. Just to check, i went into Calculator and checked what it gave me. Calculator said that the answere was 100071618270506415150156.

Edit: If you do a calculation with 15 digits(999999999999999 - 999999999999998) you get the correct answere, but when you use 16 digits, its wrong(not totally test!).