st: Nearest value in a not xtset-able panel dataset

[Francesco <cariboupad@gmx.fr>]

Dear Statalist,

I have a simple algorithmic problem that I was unable to solve so far...
Let us assume that I have the following dataset, where special_date
identifies ... a special date ;-)

ID      DATE  SPECIAL_DATE    DIFFERENCE
Jack   1               0                           missing
Jack   4               0                            -1
Jack   4               0                            -1
Jack   5               1                            0
Jack   7               0                            missing
Jack   9               0                            missing
Jack   10             0                            missing
Jack   11             0                            -1
Jack   12             1                            0
Jack   13             0                            +1
Jack   13             0                            +1
Jack   17             0                            missing

My question is :

How could I obtain the variable DIFFERENCE that gives me the
difference between the current observation's date and the nearest
special date if this distance is less than a threshold N (N=1 in the
example)?
In any case N is much smaller than the periodicity of special date.
For example N could be 10 days and Special date could occur only one
time each quarter.

Could you please help me (if possible)?

Many, many thanks in advance,

Best,

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