Finally, I include date calculations. The one I use more is the HP12c's DDAYS operation. I then set to find in the web software for it. I did not find it for the 15c, but I did for the 42s, that I then adapted. Attention - it has been modified to cater for European date format: dd.mmyyyy - you will have to go to the part where it decomposes the number into the parts and then change a few lines there.
The strategy is just to convert the date format to Julian number, convert the other date, and subtract. Fortunately, the DtoJ subroutine does not consume all 4 stack registers - or we would not be able to subtract them!
Once entered, you will still have 16 registers left. (we assume that we have set 11 registers for program use: from register 0 to .1 (so that you can enter the investment + 10 years + the number of years for the IRR program seen in past article). These 16 free registers allow you to invert a 4x4 matrix, or to solve 2 equation systems with the same coefficient matrix (a typical thing when you're solving structure problems - you apply the same set of linear equations to different conditions)
Here it is (and I have finished with it. The only possible addition would be the DATE+ function. The D->J is a part of the DDAYS program; We would just need to create the J->D - at the expense of losing necessary registers!)