Use this Google Script to setup Out of Office auto-replies in Twitter. The script reads all the Twitter @mentions and sends them a tweet with a custom status message.

/*     O U T   O F   O F F I C E   F O R   T W I T T E R    */
/*     - - -   - -   - - - - - -   - - -   - - - - - - -    */

/*     Written by Amit Agarwal http://labnol.org?p=27911    */
/*     For help, email amit@labnol.org or tweet @labnol     */

function start() {

  // Please enter dates in YYYY-MM-DD HH:MM format

  var OUTOFOFFICE_START_DATE  = "03/11/2013 18:13";
  var OUTOFOFFICE_END_DATE    = "03/11/2013 19:00";

  // This is your Out-Of-Office reply. Keep it less than 120 characters.

  var OUTOFOFFICE_TEXT = "I am currently out of the office,
                              with limited access to Twitter. Thanks!";

  // Get your Twitter keys from dev.twitter.com

  var CONSUMER_KEY     = "AAA";
  var CONSUMER_SECRET  = "BBB";

  // Change this with your Twitter handle

  var TWITTER_HANDLE   = "labnol";

  // DO NOT MODIFY ANYTHING AFTER THIS LINE

  storeKeys ( CONSUMER_KEY, CONSUMER_SECRET,
                       OUTOFOFFICE_TEXT, TWITTER_HANDLE );

  initialize ( OUTOFOFFICE_START_DATE, OUTOFOFFICE_END_DATE );

  // Make sure that Twitter oAuth is working

  doTwitter();

}


// Delete exiting Apps Script triggers, if any

function removeTriggers() {

  var triggers = ScriptApp.getScriptTriggers();

  for(var i=0; i < triggers.length; i++) {
    ScriptApp.deleteTrigger(triggers[i]);
  }

  clearDatabase();

}

function storeKeys(key, secret, text, handle) {

  ScriptProperties.setProperty("TWITTER_CONSUMER_KEY",    key);
  ScriptProperties.setProperty("TWITTER_CONSUMER_SECRET", secret);

  ScriptProperties.setProperty("OUTOFOFFICE_TEXT", text);
  ScriptProperties.setProperty("MAX_TWITTER_ID",   0);
  ScriptProperties.setProperty("TWITTER_HANDLE",   handle);

}

// Clean and Initialize the ScriptDB database

function clearDatabase() {

  var db = ScriptDb.getMyDb();
  while (true) {

    var result = db.query({});

    if (result.getSize() == 0) {
      break;
    }

    while (result.hasNext()) {
      db.remove(result.next());
    }
  }
}

// Setup triggers for the START and END dates

function initialize(start, end) {

  var startDate = new Date(start);
  var endDate   = new Date(end);

  removeTriggers();

  ScriptApp.newTrigger("autoReply")
           .timeBased()
           .at(startDate)
           .create();

  ScriptApp.newTrigger("removeTriggers")
           .timeBased()
           .at(endDate)
           .create();

}

function autoReply() {

  clearDatabase();

  ScriptApp.newTrigger("outOfOffice")
           .timeBased()
           .everyMinutes(5)
           .create();

}

function oAuth() {

 var oauthConfig = UrlFetchApp.addOAuthService("twitter");
 oauthConfig.setAccessTokenUrl("https://api.twitter.com/oauth/access_token");
 oauthConfig.setRequestTokenUrl("https://api.twitter.com/oauth/request_token");
 oauthConfig.setAuthorizationUrl("https://api.twitter.com/oauth/authorize");
 oauthConfig.setConsumerKey(
                 ScriptProperties.getProperty("TWITTER_CONSUMER_KEY"));
 oauthConfig.setConsumerSecret(
                 ScriptProperties.getProperty("TWITTER_CONSUMER_SECRET"));

}

// This function will poll twitter every 5 minutes for any @mentions

function outOfOffice() {

  oAuth();

  var twitter_handle = ScriptProperties.getProperty("TWITTER_HANDLE");

  var phrase = "to:" + twitter_handle;
  var search = "https://api.twitter.com/1.1/search/tweets.json?count=10"
             + "&include_entities=false&result_type=recent&q="
             + encodeString(phrase) + "&since_id="
             + ScriptProperties.getProperty("MAX_TWITTER_ID");

  var options =
  {
    "method": "get",
    "oAuthServiceName":"twitter",
    "oAuthUseToken":"always"
  };

  try {

    var result = UrlFetchApp.fetch(search, options);

    if (result.getResponseCode() === 200) {

      var data = Utilities.jsonParse(result.getContentText());

      if (data) {

        var tweets = data.statuses;

        if (tweets.length) {

          var db    = ScriptDb.getMyDb();
          var reply = ScriptProperties.getProperty("OUTOFOFFICE_TEXT");

          for (var i=tweets.length-1; i>=0; i--) {

            var sender = tweets[i].user.screen_name;
            var found = db.query({user: sender});

            if ( ! found.hasNext() ) {
              db.save({user:sender});
              sendTweet(sender, tweets[i].id_str, reply);
            }
          }
        }
      }
    }
  } catch (e) {
    Logger.log(e.toString());
  }
}

// If an @mention is found, send an Out of Office tweet to that user.

function sendTweet(user, reply_id, tweet) {

  var options =
  {
    "method": "POST",
    "oAuthServiceName":"twitter",
    "oAuthUseToken":"always"
  };

  var text   =  "@" + user + " " + tweet;
  text   =  encodeString(text.substr(0, 140));

  var status = "https://api.twitter.com/1.1/statuses/update.json"
             + "?status=" + text + "&in_reply_to_status_id=" + reply_id;

  try {
    var result = UrlFetchApp.fetch(status, options);
    ScriptProperties.setProperty("MAX_TWITTER_ID", reply_id);
  }
  catch (e) {
    Logger.log(e.toString());
  }

}

function doTwitter() {

  oAuth();

  var req = "https://api.twitter.com/1.1/application/rate_limit_status.json";

  var options =
  {
    "method": "get",
    "oAuthServiceName":"twitter",
    "oAuthUseToken":"always"
  };

  try {

    var result = UrlFetchApp.fetch(req, options);

  } catch (e) {
    Logger.log(e.toString());
  }
}


function encodeString (q) {
   var str =  encodeURIComponent(q);
   str = str.replace(/!/g,'%21');
   str = str.replace(/\*/g,'%2A');
   str = str.replace(/\(/g,'%28');
   str = str.replace(/\)/g,'%29');
   str = str.replace(/\'/g,'%27');
   return str;
}