With this function you can check two dates to see which one is the earlier or later than the other. The function returns -1 if date1 is earlier, 0 if both dates are the same, and 1 if date1 is later.
date_compare_date( date1, date2 );
|date1||The first date.|
|date2||The date to compare it to.|
d = date_compare_date(date_create_datetime(2011, 9, 15, 11, 4, 0), date_current_datetime());
This would set "d" to the corresponding value depending on which of the dates was the earliest, likely 1 since the current date would be further ahead than 15th September 2011.