The <A HREF=http://www.gnu.org/software/glpk/glpk.html>GNU MathProg modeling language </A> is an open source modeling language that implements a subset of the AMPL language
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Significant editing of the AMPL model and data will be required because the implementation is a subset of AMPL at the time of this writing. 
Here is one example: In order to use the GNU MathProg language for the AMPL model, the time set should be given as integers, as in
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    set TT := 1..8;
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in which case ord(t) = t.
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On printing you can convert elements of TT to calendar dates as
follows:
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set TT := 1..8;
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param START, symbolic, := "1jan04";
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printf{t in TT} "...%s...",<br>
   time2str(str2time(START, "%d%b%y")+86400*(t-1), "%d%b%y");