You’re comparing strings when you really want to be comparing datetimes.
So instead of converting the current time into a string, you should convert today’s 21:30 into a datetime.
As @bmusson mentioned, you're doing a comparison on Strings here, and the expression language will attempt to coerce these to numbers before giving up all together.
In this case your comparison ends up being 21.0 < 21.0, resulting in false.