this.lastFreeLifeTS = ( new Date() ).time;

var currTime:Number = ( new Date() ).time;
var diff:Number = ( currTime - this.lastFreeLifeTime );
var lifeTime:Number = 1000 * 60 * 20; // give a life every 20 minutes
if( diff > lifeTime )
{
    giveFreeLife();
    this.lastFreeLifeTime += lifeTime;
}

// starting off
var lifeTime:Number = 1000 * 60 * 20; // 20 minutes
var msUntilNextLife:Number = ( ( new Date() ).time - this.lastFreeLifeTime ) + lifeTime;
this.nextFreeLifeTime = getTimer() + msUntilNextLife;

...

// check if we need to give a new life
if( getTimer() > this.nextFreeLifeTime )
{
    giveFreeLife();
    this.lastFreeLifeTime += lifeTime;
    this.nextFreeLifeTime += lifeTime;
}

// load our SharedObject/XML and set our lastFreeLifeTime
...

// check how much time has passed and give any lives necessary
var currTime:Number = ( new Date() ).time;
var diff:Number = currTime - this.lastFreeLifeTime;
var lifeTime:Number = 1000 * 60 * 20; // 20 minutes
var numLives:int = int( diff / lifeTime );
if( numLives > 5 )
    numLives = 5; // only give a maximum number of lives
if( numLives > 0 )
{
    giveFreeLives( numLives );

    // update our lastFreeLifeTime only if we didn't give the max number of lives
    // (as otherwise we don't need it)
    // NOTE: because we're adding (lifeTime * numLives), it handles the time between
    // lives nicely. E.g. If we get a life every 20m and we come back after 30m, then
    // our lastFreeLifeTime will be (20 * i), meaning a 10m difference between the
    // current time, meaning we only have to wait 10m to get our next one
    if( numLives < 5 )
        this.lastFreeLifeTime += lifeTime * numLives;
}

<?php
    echo round( microtime( true ) * 1000 );
?>

// NOTE: add all the other listeners, like IOErrorEvent, and SecurityErrorEvent
var urlLoader:URLLoader = new URLLoader( new URLRequest( "http://myserver.com/timestamp.php" ) );
urlLoader.addEventListener( Event.COMPLETE, onGetTime );

function onGetTime( e:Event ):void
{
    // remove our event listener so we can clean up
    var urlLoader:URLLoader = ( e.target as URLLoader );
    urlLoader.removeEventListener( Event.COMPLETE, onGetTime );

    // the data is the timestamp returned by php, as a string, so convert it
    var timestamp:Number = Number( urlLoader.data );
    if ( isNaN( timestamp ) )
    {
        trace( "Couldn't get the timestamp from the server! Returned details: " + urlLoader.data );
        timestamp = ( new Date() ).time; // fallback to client
    }

    // do something with the time
}