How to get the difference between one row and next row for one column in netezza

I am using Netezza for windows. And I tried to get the get the difference between one row and next row  for one column.

I found the function LAG and LEAD.

Here is the query:

select extract(epoch from a.day_id) - extract(epoch from a.lag1) as diff1,day_id from (
select day_id, lag((day_id),1, null) over (order by day_id) as lag1 from table1 where day_id is not null group by day_id ;