Gruntfile.js

module.exports = function(grunt) {

  grunt.initConfig({
    
    // [other stuff...]
    
    compass: {
      options: {
        sassDir: 'sass',
        javascriptsDir: 'javascripts',
        fontsDir: 'fonts',
        outputStyle: 'compressed',
        environment: 'production'
      },
      rTheme: {
        options: {
          importPath: 'sass/r-only',
          httpPath: '/wp-content/themes/r/',
          cssDir: 'stylesheets',
          imagesDir: 'img'
        }
      },
      fTheme: {
        options: {
          importPath: '../f/sass/',
          cssDir: '../f/stylesheets',
          imagesDir: '../f/img',
          httpPath: '/wp-content/themes/f/'
        }
      }
    },
    // https://github.com/pajtai/grunt-php-set-constant
    setPHPConstant: {
      styles: {
        constant: 'R_STYLES_UPDATED',
        value: '<%= grunt.template.today("yyyymmddHHMMss") %>',
        file: 'functions.php'
      }
    },

  // [other stuff...]

  // Tasks
  // Running `grunt styles` at the command line runs `compass` and `setPHPConstant` as defined above 
  grunt.registerTask('styles', ['compass', 'setPHPConstant:styles']);

};

/*
Then, later on:

wp_register_style( $handle, $src, $deps, R_STYLES_UPDATED );
*/