I'm not sure why ppl downvoted you. But from what I've heard - never implemented myself - what you're saying is common... At least as far as using separate fixed point values to represent the left and right sides of the decimal place.

Now I'm sure for things like computing interest by raising things to a power or even computing percentages, etc, floating point is used (and eventually reduced to fixed point values), so maybe someone found it too absolute to say "all" financial math is fixed point? Anyway, if someone with experience can confirm or deny, it would be neat...

It's absolutely not. Everything I've seen has been 64-bit double. If you need to, you round either intermediate or final calcs to a set precision.

I once worked on replicating a particular index, and the engine used fixed point internally. In order to get the values to match with the ones published officially, we had to emulate Excel floating point: our fixed point values wouldn't line up with theirs.

I think there's internal rounding that hides it. Try A1=1e50, B1=A1+1, and C1=B1-A1.

Excel definitely used floating point in the past (because you used to be able to use it to test for the Pentium FDIV bug) and according to the documentation, it still does: https://support.microsoft.com/en-us/kb/78113