This gist cleans your OS X trash by permanently removing items trashed more than X days ago

clean_trash.js

#!/usr/bin/env osascript -l JavaScript
var numDaysToKeep = 20;
var now = $.NSDate.date;
console.log("Now:", now.description.js);
var deleteThreshold = now.dateByAddingTimeInterval(-86400 * numDaysToKeep);
console.log("Will delete items whose date added is earlier than", deleteThreshold.description.js);

var urlPath = $.NSURL.alloc.initFileURLWithPathIsDirectory($('~/.Trash').stringByExpandingTildeInPath, true);
var fm = $.NSFileManager.defaultManager;
var urlContents = fm.contentsOfDirectoryAtURLIncludingPropertiesForKeysOptionsError(urlPath, ["NSURLAddedToDirectoryDateKey"], $.NSDirectoryEnumerationSkipsHiddenFiles, null).js;
urlContents.forEach(function (url) {
    var date = $();
    url.getResourceValueForKeyError(date, "NSURLAddedToDirectoryDateKey", null);
    console.log("Found file", url.fileSystemRepresentation, "with date added =", date.description.js);
    if (date.compare(deleteThreshold) === $.NSOrderedAscending) {
        console.log("Will delete", url.fileSystemRepresentation);
        fm.removeItemAtURLError(url, null);
    } else {
        console.log("Will keep", url.fileSystemRepresentation);
    }
});

st.qzy.cleantrash.plist

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
	<key>Label</key>
	<string>st.qzy.cleantrash</string>
	<key>ProcessType</key>
	<string>Background</string>
	<key>ProgramArguments</key>
	<array>
		<string>/path/to/cleantrash</string>
	</array>
	<key>StandardErrorPath</key>
	<string>/path/to/cleantrash/output</string>
	<key>StandardOutPath</key>
	<string>/path/to/cleantrash/output</string>
  <key>RunAtLoad</key>
  <true/>
	<key>StartCalendarInterval</key>
	<dict>
		<key>Hour</key>
		<integer>0</integer>
		<key>Minute</key>
		<integer>0</integer>
	</dict>
</dict>
</plist>