Here is my problem

Worker date
01 01-jan-2007
01 15-jan-2007
01 15-feb-2007
01 28-mar-2007
01 28-apr-2007

I want to find when worker 01 has a date span greater than 30 days

SELECT date,date0 FROM (

SELECT  worker,date, (
 SELECT max(t1.date) FROM t t1 WHERE t1.date<t.date
) date0
FROM t t) v WHERE datediff(date,date0)>30;

The above is not a readily functional solution, I use it merely to illustrate the idea.
I set the dialect to MySQL as the above uses the handy datediff() function.

Let me know if that helped you. Also, please do not hesitate to ask if you have any questions.

Best Regards,
Dimitar