Please I need to calculate the difference between a date n and the first date of a long set.
I do not know how to specify the date 1

example:
id . date_edss edss
Cemcat-10 04/01/2000 1.5
Cemcat-10 27/12/2000 1
Cemcat-10 22/12/2001 0
Cemcat-10 17/12/2002 1
Cemcat-10 12/12/2003 1
Cemcat-10 06/12/2004 0
Cemcat-10 01/12/2005 1
Cemcat-10 26/11/2006 1
Cemcat-10 21/11/2007 1
Cemcat-10 15/11/2008 1
Cemcat-10 05/11/2010 0
Cemcat-10 20/10/2013 0

excuse me, I'm new to the list.
thansk