use of datediff and/or timediff

[R L]

Hello All,
I'm in the process of converting my MS SQL statements to MySQL 4.1.3 beta and I've come up against a little problem.

I have a query that queries a members table to get a count of members that have logged in in the last 15 minutes. My current query is below...

select count( memberid )
from members
where datediff( n ,l astlogin, now() ) < 15

How can I do this or something similar with mySQL??

I've looked at the datediff and timediff functions, but can't figure out how to use them in a similar fashion.

Thanks in advance!