Victor,

With issue2736, you included a variant of this patch where you use -1 as a sentinel value for tm_wday.  I think this is a better choice. (The answer to the ultimate question of life is not universally known.:-)

On a more serious note, is checking for the -1 return value necessary?  I would think a check for changed tm_wday would be enough.

Finally, a coding style nitpick: don't use superfluous parenthesis in logical expressions.