Thanks for your bug report. This is related to rounding/precision/etc. The result you are seeing in the latest version of mIRC is the expected, albeit wrong, result.
mIRC is a 32bit application. For it's $calc()/$base()/scripting calculations it can handle a maximum of a double value. IEEE 64-bit double precision provides exactly 53 bits of mantissa. The remaining bits are for the sign and exponent. So a 64bit value cannot fit in a double value.
However, the fact that a twenty year old version of mIRC, v6.03, returned the correct result, is intruiging. If I use the exact same code from v6.03 in v7.69, I still see the same incorrect result in v7.69. Which is what mIRC should be returning: the correct incorrect result ;-)
mIRC v6.1 returns the same result as v7.69. So I assume that it was the switch from an older Visual C++ compiler, used for v6.03, to the newer Visual C++ .Net used for v6.1 that resulted in this change. My guess is that Microsoft fixed an issue in their floating point library to make it comply with IEEE double precision.
Update: And there you go:
https://stackoverflow.com/questions/7120710/why-did-microsoft-abandon-long-double-data-type
