What's the hardest (longest-running) financial calculation?

[EdS2]

Thanks for the answers, everyone. So, finding an interest rate in a Future Value of Money problem is hard, and finding the Internal Rate of Return for a cashflow is (or might be) harder.

Valentin, your comment led me to some specific articles in HP Journal, which might be of interest to other readers (I'm sure you are already familiar with them) and which contain some sample calculations:

Printing Financial Calculator Sets New Standards for Accuracy and Capability (HP-92) October 1977, page 22

A Handheld Business Consultant (HP-18C) August 1987, page 4
"Because the Business Consultant uses the same CPU as the HP-71B Handheld Computer, its financial calculations run at least 15 times faster than those on the HP-12C."

I looked for material on the original HP-12C, but the only HP Journal article I found which directly mentions the Voyager machines is later, about the 15C:

Scientific Pocket Calculator Extends Range of Built-in Functions May 1983, page 25

and a short section on the HP-12C emulation available for the HP 150 touchscreen computer:
Financial Calculator August 1984, page 22

There's also some interesting reading in HP Solve magazine, issue 25 from September 2011, about the 12C:
"...different financial calculators display different amounts. Here is the result if you use an HP-27, HP-92, HP-37, HP-38, and HP-12C = $331,667.0067. As an aside, the new HP 10bII calculates $331,667.006691."