As has been mentioned (thank you, gentlemen!), you are not using Java datatypes. That being said, Starting with v7.8.something, there are a whole slew of functions added under system.date.* that use java data types.
Your script can now be shortened to something like:
dateSelector = event.source.parent.getComponent('Popup Calendar')
now = system.date.now()
if system.date.isAfter(dateSelector.date , now):
dateSelector.date = now