calculating date differences based on a conditionality

I am trying to calculate the date difference between 2 different tests (namely AST and THR here):

21/06/2005 AST 21/06/2005 THR 30/08/2005 THR 07/04/2004 THR 10/09/2004 THR 13/09/2004 AST 04/06/2004 THR

what I want: the difference in days between the 2 closest pairs of AST/THR tests. I have tried reshaping the data, but since I also have test values, I can only reshape for one and not the other (date or lab test result). IT is a very large database, and I also have them divided by patient ID. Any suggestions?