st: How to record the first occurrence of the event?

[Roxannie Wankel <roxanniew@gmail.com>]

how to record the first occurrence of the event?

A list of firms established one or several joint ventures in the
period of 1990-2009. I need to record the first joint venture that a
firm established witin three years.

For example,
Company A established a joint venture with Company B on Jan 5, 1991;
(joint venture 1)
Company A established another joint venture with Company C on Feb 9,
1993. (joint venture 2)
Company A established the third joint venture with COmpany D on Dec 5,
1994. (joint venture 3)
Company A establisehd the fourth joint venture with Company E on Dec
6, 2005 (joint venture 4)

Then I have to record the first and the fourth joint venture.
I should not record the second and the third one because the second
one is within 3 years of the first one; and the third one is within 3
years of the second one.

How can I use the STATA command to record these two joint ventures?
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