Count all records from midnight until now()

Count all records from midnight until now()

I'm trying to grab all records created from midnight of the current day
until the time a button is pressed.
I've tried something like this meanwhile but it doesn't really solve the
issue.
SELECT COUNT(*) FROM ... AND a.lastAction > now() - interval '24 hour'"
What I need is something like:
SELECT COUNT(*) FROM ... AND a.lastAction > midnight() AND a.lastAction <
now()
Is there a way to convert a string like "2013-08-06 00:00:01" to a timestamp?