Because due dates are computed by the Business Engine using task durations, there is no ability to set due dates directly/dynamically. As for having this ability in a future release, please do submit this as an enhancement request. At the present time, there are no current plans to modify the Business Engine near term.
For example they select 10th of august, the task duration we know is 5 days so we set the hidden field using JS to 5th of August. We then set the scheduled start date of the task to this date, the task would then trigger 5 days before the due date and the due date would be that which the suer had selected in the first place.
The problem I have here is with date objects in the catalog. When we grab data from a date field using JS it is as a string, we then convert it to a date object using JS and then do the manipulation on it (i.e. minsu 5 days or whatever) we then normally set it back to a string value and set the value of the date field to that string value in the normal format. The problem here is that when we use the scheduled start date on the option and use that field as the data, it sdoesn't work as it is in a sense a string value which the scheduled start date does nto recognise. Should we just elave it as a date object when putting it back in to the field and will the scheduled start date recognise this as a date and thus schedule the task accordingly or is there some toher format this data must take? So far I haven't been able to get this to work.