st: generating a seq() going backwards and forward from a given timepoint?

["Ariel Linden, DrPH" <ariel.linden@gmail.com>]

Hi Listers,

This is probably a very simple solution, but I am getting stuck. I'd like to
generate a running sequence going both backwards and forwards from a
specified timepoint. -seq()- is pretty straightforward (pun intended) for
going forward, but I can't seem to figure out the backwards part.

Below I have and data with the "period" variable that I'd like to achieve
via code (I did this manually). For this example, I specified that
01/01/2012 is the starting point and ran the following code:

. display date("01/01/2012", "MDY")
18993

. bys ID: egen period = seq() if date>=18993
(48 missing values generated)

How do I get the backward periods (-1 to -24)? There will be IDs with
varying numbers of periods available, so this must work via bysort ID,
anchored on the same starting date for everyone:

ID	date	period
WC9	01/01/2010	-24
WC9	02/01/2010	-23
WC9	03/01/2010	-22
WC9	04/01/2010	-21
WC9	05/01/2010	-20
WC9	06/01/2010	-19
WC9	07/01/2010	-18
WC9	08/01/2010	-17
WC9	09/01/2010	-16
WC9	10/01/2010	-15
WC9	11/01/2010	-14
WC9	12/01/2010	-13
WC9	01/01/2011	-12
WC9	02/01/2011	-11
WC9	03/01/2011	-10
WC9	04/01/2011	-9
WC9	05/01/2011	-8
WC9	06/01/2011	-7
WC9	07/01/2011	-6
WC9	08/01/2011	-5
WC9	09/01/2011	-4
WC9	10/01/2011	-3
WC9	11/01/2011	-2
WC9	12/01/2011	-1
WC9	01/01/2012	1
WC9	02/01/2012	2
WC9	03/01/2012	3
WC9	04/01/2012	4
WC9	05/01/2012	5
WC9	06/01/2012	6
WC9	07/01/2012	7
WC9	08/01/2012	8

Thanks!

Ariel

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