st: vertical (within a column) calculations

["Mendoza Aldana, Dr Jorge Antonio (WPRO)" <MendozaAldanaj@wpro.who.int>]

Dear all,
using Stata 10.1
I have a dataset with place, date and a variable lab that can take values from 1 to 4, and also missing values. I would like to generate a variable containing, for each place, the difference between the date of a record with lab==1 (my reference record within a place) and the date of the other records (within the same place).

I will appreciate very much if you could indicate the function(s) for doing this or the link dealing with.

Thank you very much in advance
Jorge

------ Data example -----------
PLACE     DATE    LAB
A	04 Mar 10	.
A	16 Mar 10	2
A	17 Mar 10	1
A	19 Mar 10	2
A	23 Mar 10	2
A	29 Mar 10	2
A	30 Mar 10	2
A	30 Mar 10	2
A	31 Mar 10	1
A	31 Mar 10	1
A	01 Apr 10	.
A	01 Apr 10	.
A	01 Apr 10	2
A	02 Apr 10	2
B	07 Jun 10	1
C	25 Jun 10	2
D	07 Feb 10	.
D	03 Mar 10	.
D	01 Apr 10	1
D	03 Apr 10	1
D	04 Apr 10	1
D	04 Apr 10	1
D	07 Apr 10	1
E	24 Jun 10	1
F	02 Apr 10	.
F	09 Apr 10	2
F	10 Apr 10	.
F	27 Apr 10	1
F	07 Apr 10	2
F	22 Apr 10	.
F	26 Mar 10	.

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