st: st: generating lag variable in a Panel Dataset

["Clifton Chow" <clifton_chow@post.harvard.edu>]

I have a panel dataset on employment in which individuals progress between jobs, each with a job start and end date as follows:

ID     JobNumber    Start          End
1              1           8/1/10      12/30/10
1              2           1/20/11      3/10/11
1              3           5/25/11      12/20/11
2              1           11/1/10       3/20/11
2              2            4/1/11        9/30/11 

I am trying to calculate the duration of Unemployment by lagging job end date so that unemployment duration is the date difference between the end of the previous job and the start of the following job.  However, I need to carefully generate the lag variable by ID.  Is there a way I could do this easily within STATA?  For example, something like 

gen unempdur=Start - lag(End) if ID = lag(ID)   ?

Thanks for your replies.
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