Set a view filter programmatically
Set the default value to now using GUI. This should give you what you need without any code.

If you need to use fiscal years, either create own custom filter, one that will translate real date to fiscal year. Read manual and more detailed manual to see how to do it. Simply changing default value in date will not be enough, as user may inject date with more than a year set, making built-in filter not work the way you want it to. And then, well, then use now as default value.

You probably want to inherit from class views_handler_filter_date and override it's extra_options() method