The Calendar FAQ includes a number of formulas for calculating various calendar values.

I have written a very simple implementation of these formulas in the C language. Rather than providing the most efficient code, the implementation intentionally follows the formulas very closely.

The code can be downloaded as a ZIP file

As described the Calendar FAQ, February had 30 days in Sweden in 1712.

The picture below shows two pages from a Swedish almanac from 1712.

Although a 30 February has indeed occurred in history, what about 31 February?