Andrew is right, but only Excel 2002 and prior can read wk1 files. Everyone else stupidly abandoned converting the platform. If you work with local governments or even the Federal Government in the USA, you still need to submit data to them in wk1 format. Smartest answer is to shop around for Lotus 1-2-3 DOS versions 2.x and Funk Software DOS version Worksheet Utilities, because Excel and all other Windows-platform spreadsheets are less functional. Excel 2002 and prior cannot itself program the wk1 functions, but can READ and CALCULATE the functions, if you first do your worksheet in DOS.
I don't know whether the MAC OS can read wk1 files, but I doubt it, since MAC uses Windows products for its own Office stuff. So ask Apple.
Latest Windows platform on which you can comfortably use a DOS window, is XP Pro SP3. It might be possible to run a DOS program in 32-bit Windows, I'm not sure. According to MS Answers forum, you'd have to get DOSBOX to run in Win7 64-bit. I have no idea how valid that is, since I refuse to upgrade. Am only reporting what I found in Google.
Win8 is an arcane interface over Win7, so maybe the foregoing paragraph will also be true for Win8. Again, I'm only guessing, as I refuse to upgrade.