WHERE clause with timestamp data type

The following command works great,
SELECT AVG(column) FROM table WHERE timestamp > 2007-08-23;

But when I try it with finer resolution (hour, minutes, seconds) like this, I get a syntax error
SELECT AVG(heartrate) FROM heartrates WHERE timestamp > 2007-08-23 19:48:09;

Do I need to convert from string format to total number of seconds? Any ideas on how to get this work?

Thanks!
Chirag