getting the time from an sql DateTime field

I need to get the rows from a table which has a datetime attribute. The datetime attribute gives the date and time of insertion of a given row. I want to get the rows which have been inserted say within the last five seconds.To do this i have to get the time of the datetime attribute.
Assume that the date is irrelevant since the table will only hold the insertions within the current day.
Is there a simple way of doing this?