//echo -a $calc(100000000000000000000000000000000000000000000000000000000000 + 1)

returns

99999999999999997000000000000000000000000000000000000000000

...

wow hope i never need to add 1 to 100000000000000000000000000000000000000000000000000000000000

well according to the post i tried the following:

//echo -a $calc(10000000000000000000000 + 0)
it returns 10000000000000000000000
which is perfectly fine.

but when i try to add another 0 behind
//echo -a $calc(100000000000000000000000 + 0)
it returns 99999999999999992000000

seems like the problems only affect with int numbers bigger than 24char

so crazy, isn't it?

Yes, as with all programming/scripting languages, $calc() handles numbers up to a certain size, anything beyond that will result in overflow and the result is indeterminate.

and it's surprising it can handle so big numbers