With this function you can check two dates and times to see which one is the earlier or later than the other. The function returns -1 if date1 is earlier, 1 if date1 is later and 0 if they are the same, although since this takes time into account as well, a 0 for matching is very rare (it literally has to be the same second of the same day of the same month of the same year to return a 0).
|date1||The first datetime.|
|date2||The datetime to compare it to.|
d = date_compare_datetime(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 datetimes was the earliest, likely -1 since the current date would be further ahead than 15th September 2011.