The Firefighter Calendar is one in a long series of computerized platoon-based calendars by T. Wesley Erickson.
The first was written for the HP-41 hand-held calculator; it was published in the PPC Journal in August of 1984.
Shortly thereafter, it was published as a macro for Lotus 1-2-3 release 1A (in the Safetynet Forum of Compuserve) to determine the platoon on duty for a specified date. When Lotus added string-manipulation functions in release 2.0, a macro was no longer required, and it was published as a function that could be stored in any cell of a Lotus 1-2-3 worksheet.
The first standalone DOS version was written in Turbo Pascal version 2.0; DOS versions used "cloner" technology to store shift rotation information in the executable. Unfortunately, with the rise of computer viruses (and virus- detection software), this technique triggered "virus-like behavior" messages, causing end-users no small amount of concern. Subsequent versions have used Windows .ini files to store configuration information.
Specialized versions have been written for the Hewlett-Packard HP110 Portable and Portable Plus computers, including a version on ROM (as part of a collection of useful DOS utilities), as well as versions for Apple and CP/M computers.
When Borland released Turbo Pascal for Windows, the program was ported to a graphical user interface.
The current release (version 6) is written in Delphi 10.4 Sydney (Embarcadero RAD Studio 10.4).