Time-limited trials are difficult to do foolproofly. You
could write the start date to the encrypted local store. The ELS
isn't automatically deleted when an app is uninstalled, but a savvy
user could delete it manually and then reinstall your app to reset
the trail period.
A feature-limited trial is easier, as you could just provide
an AIR file containing a limited version which can be upgraded to
the full version when the user buys it.