Function written for http://stackoverflow.com/a/31277867/1677912

lastModified.gs

/**
 * Return the time that the referenced cell or range was last changed.
 * Initial use shows current time.
 *
 * @param {Sheet3!B32}  reference   Cell or range to monitor.
 * @returns                         The time the reference was last changed.
 * @customfunction
 */
function lastModified( reference ) {
 // From gist.github.com/mogsdad/3bef949fcae5b4411524
 // Surprise - we don't actually care what has changed!
 return( new Date() );
}