SQL: Calculating Number of Days Between Dates of One Column In Different Rows

Question

With my data I have individuals taking an assessment multiple times at different dates. It looks something like this:

╔════════╦═══════════╦═══════════╦═══════╗
║ Person ║ ID Number ║ Date      ║ Score ║
║ John   ║ 134       ║ 7/11/2013 ║ 18    ║
║ John   ║ 134       ║ 8/23/2013 ║ 16    ║
║ John   ║ 134       ║ 9/30/2013 ║ 16    ║
║ Kate   ║ 887       ║ 2/28/2013 ║ 21    ║
║ Kate   ║ 887       ║ 3/16/2013 ║ 19    ║
║ Bill   ║ 990       ║ 4/18/2013 ║ 15    ║
║ Ken    ║ 265       ║ 2/12/2013 ║ 23    ║
║ Ken    ║ 265       ║ 4/25/2013 ║ 20    ║
║ Ken    ║ 265       ║ 6/20/2013 ║ 19    ║
║ Ken    ║ 265       ║ 7/15/2013 ║ 19    ║
╚════════╩═══════════╩═══════════╩═══════╝

I'd like it to have another column at the end that calculates the number of days since the first assessment for that person. I'd also settle for the number of days since the previous assessment for that person if that's easier.

Ideally it would look like this:

╔════════╦═══════════╦═══════════╦═══════╦══════════════════╗
║ Person ║ ID Number ║ Date      ║ Score ║ Days Since First ║
║ John   ║ 134       ║ 7/11/2013 ║ 18    ║ 0                ║
║ John   ║ 134       ║ 8/23/2013 ║ 16    ║ 43               ║
║ John   ║ 134       ║ 9/30/2013 ║ 16    ║ 81               ║
║ Kate   ║ 887       ║ 2/28/2013 ║ 21    ║ 0                ║
║ Kate   ║ 887       ║ 3/16/2013 ║ 19    ║ 16               ║
║ Bill   ║ 990       ║ 4/18/2013 ║ 15    ║ 0                ║
║ Ken    ║ 265       ║ 2/12/2013 ║ 23    ║ 0                ║
║ Ken    ║ 265       ║ 4/25/2013 ║ 20    ║ 72               ║
║ Ken    ║ 265       ║ 6/20/2013 ║ 19    ║ 128              ║
║ Ken    ║ 265       ║ 7/15/2013 ║ 19    ║ 153              ║
╚════════╩═══════════╩═══════════╩═══════╩══════════════════╝

Answer 1

select  *
,       datediff(day, min(Date) over (partition by [ID Number]), Date)
from    YourTable

Live example at SQL Fiddle.

Answer 2

I like Andomar's answer, but if you wanted to find both days between and total days since first you could do this:

SELECT a.*
        ,ISNULL(DATEDIFF(day,b.Date,a.Date),0)'Since Previous'
        ,datediff(day, min(a.Date) over (partition by a.[ID Number]), a.Date)'Since First'
FROM (select  *,ROW_NUMBER() OVER(PARTITION BY [ID Number] ORDER BY DATE)RowRank
      from    YourTable
      )a
LEFT JOIN (select  *,ROW_NUMBER() OVER(PARTITION BY [ID Number] ORDER BY DATE)RowRank
      from    YourTable
      )b
ON a.[ID Number] = b.[ID Number]
 AND a.RowRank = b.RowRank + 1

Demo: SQL Fiddle