Preparation time PRPTIME not working on weekends

I found a problem in X3, i don't know if it is related to PARAMETERS or bug...

In our company we have preparation time setted in all operations representing the "Transportation time" in each operation. Normally we have 2 days in preparation time and in some cases 1 day after operation.

The fields we use are:

       PRPTIME   :  Time before the operation

       PSPTIM      : Time after the operation

The problem is that, if the operations dates are in week days, the system do it well and calculate the starting date using that times, but if the first operation is on a monday, the preparation time doest not appear. Example:

Planned in weekdays:

Planned on monday (hidden preparation time in weekend)