The ephemeris for the calendar is computed using the C code in the libastro subdirectory of the source release of XEphem 3.1. This excellent resource is maintained by Elwood Downey.

The routines of libastro are wrapped using the SWIG simplified wrapper and interface generator developed and maintained by Dave Beazly. A SWIG interface file, astro.i, was constructed by reading through the header files for libastro and making a SWIG interface description for each macro or subroutine that appeared useful. Comments were pulled from the source files and inserted into the interface file to document the routines. SWIG then produced this documentation file for the wrapped library.

The SWIG wrapper and the libastro library itself were linked into a shared library, libastro.so, which could be dynamically loaded into Tcl/Tk, a scripting language and graphical user interface toolkit originally developed by John Ousterhout. Tcl/Tk is currently being maintained as an Open Software project.