How can I move the date from right (default) to left side in a letter in LaTeX?

One idea is to just flush the date to the left by redefining date, or actually @date. (It requites that this is the only place @dateis used.)

documentclass{letter}
usepackage{amssymb,amsmath}
usepackage{graphicx}
oddsidemargin=.2in
evensidemargin=.2in
textwidth=5.9in
topmargin=-.5in
textheight=9in


newcommand {qed}{mbox{$Box$}}
renewcommand {iff}{Longleftrightarrow}
newcommand {R}{mathbb{R}}
newcommand {N}{mathbb{N}}
newcommand {Q}{mathbb{Q}}
newcommand {Z}{mathbb{Z}}

newcommand {sub}{mbox{SB}}

%%%%%%%%%%%%%% New stuff
makeatletter
renewcommanddate[1]{gdef@date{hbox to linewidth{#1hss}}}
makeatother
date{today}
%%%%%%%%%%%%%%

begin{document}
begin{letter}{Mathematics Search Committee
Department of Mathematics
University of Somewhere
1234 Math Building
Somewhere, IL, 23456}


opening{Dear Search Committee,}

My research interests include continuous logic and its application to metric structures from analysis and geometry. My thesis work is on the model theory of $R$-trees, which are metric spaces of central importance in geometric group theory. I am currently finishing a paper with my advisor, and expect at least one more paper to result from the contents of my thesis.

closing{Sincerely, }
Sylvia Carlisle
end{letter}

end{document}