env.Command('foo.out', 'foo.in', "dateVar=$(date +%F-%k-%M); sed -i "s/.txt/.txt?v=$dateVar/g" example.properties");

env.Command('foo.out', 'foo.in',
            'dateVar=$$(date +%F-%k-%M); '
            'sed -i "s/.txt/.txt?v=$$dateVar/g" example.properties');