TIMESTAMPDIFF between two status

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Hello, 

I would like to calculate the number of days between two date from CSW status; 

I tried this formula : 

TIMESTAMPDIFF(SQL_TSI_DAY,(CASE WHEN "Submission CSW Status - Historical"."Historical Status Name" = 'A évaluer' THEN "Submission CSW Status - Historical"."Submission Historical Status Start Date" ELSE NULL END),(CASE WHEN "Submission CSW Status - Historical"."Historical Status Name" = 'Satisfait les qualifications de base' THEN "Submission CSW Status - Historical"."Submission Historical Status Start Date" ELSE NULL END))

 

But it seems does not work, I have no values. 

Do you know where is my error? Do you have an idea how can I try to obtain this number ?