Assuming that "Fecha" is a Date Type variable then the problem is likely to be the order of data assignment.
You cannot rely on the data assignments happening in the order you enter them. Never assign a variable and the use the variable in the same data assignment step. Split the data assignment in to 2 or more different steps.
In step one assign the string variable. Then in a seperate step convert the vaiable to a date, then in a final step assign it to the index field.
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