Unlike Word I actually spent a few years of my life working on this.
At the surface layer this era of Excel ("BIFF" documents) isn't too bad, getting say, a table of small integers representing people's annual salaries out of an XLS file is very do-able and many programs today will get that right.
As you start to dig down it gets nastier pretty quickly. Formulae require implementations that match not just what Microsoft's published documents (I have loads of these on a shelf I rarely look at now) say, but what Excel actually did, bug for bug, back in the 1990s. Maybe the document says this implements a US Federal tax rule, but alas Excel got the year 1988 wrong, so actually it's "US Federal tax rule except in 1988".
You also run into show stoppers that prevent the oft-imagined "Just transform it to some neutral format" because Excel isn't a typed system. What is 4? Did you think it's the number 4? Because the sheet you're trying to parse assumes it's actually the fourth day of the Apple Macintosh epoch in one place, but in another place uses it to index into an array. Smile!
Finally in complicated sheets (often "business critical") there's a full-blown Turing complete programming language, complete with machine layer access to the OS. Good luck "translating" that into anything except an apologetic error message.
At the surface layer this era of Excel ("BIFF" documents) isn't too bad, getting say, a table of small integers representing people's annual salaries out of an XLS file is very do-able and many programs today will get that right.
As you start to dig down it gets nastier pretty quickly. Formulae require implementations that match not just what Microsoft's published documents (I have loads of these on a shelf I rarely look at now) say, but what Excel actually did, bug for bug, back in the 1990s. Maybe the document says this implements a US Federal tax rule, but alas Excel got the year 1988 wrong, so actually it's "US Federal tax rule except in 1988".
You also run into show stoppers that prevent the oft-imagined "Just transform it to some neutral format" because Excel isn't a typed system. What is 4? Did you think it's the number 4? Because the sheet you're trying to parse assumes it's actually the fourth day of the Apple Macintosh epoch in one place, but in another place uses it to index into an array. Smile!
Finally in complicated sheets (often "business critical") there's a full-blown Turing complete programming language, complete with machine layer access to the OS. Good luck "translating" that into anything except an apologetic error message.