Algorithm for "1st Tuesday", "last Saturday", etc.

Can someone point me to an algorithm that, given a speific month and year,
can convert "dates" such as "1st Tuesday", "last Saturday", etc. to actual
numeric dates.

I've tried a (DOS-based) executable called "DATES.EXE", but it doesn't
handle "even" and "odd" years (which I use for tax purposes) so I wrote
a PRE-processor, and I don't like its output format so I wrote a POST-
processor. If I could find the appropriate algorithm as described agove,
it would probably just be easier to write one program!
--
--Myron A. Calhoun.
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.