Time is a data unit dimension and as the calculations run per data units as a default you cannot and should not write outside of the current data unit. However, you can retrieve values from outside of the current data unit. So, I think you could first calculate the value you want to spread to the future month and then you would need to run calculations for the prior months to retrieve the calculation outcome to the prior months. This could be done with custom calculate for example.
Hopefully this answered you question.