I've done something similar, although usually I am using DateTime widgets as service inputs and use expressions to set the default selections. But this will work for direct service inputs as well.
My suggestion is to have two expressions.
Set them to have DateTime outputs.
Expression1's expression would be: new Date()
and Expression2's expression would be: new Date(new Date().getTime() - 7776000000)
Then make sure to wire the outputs of Expression1 to the service's endDate and Expression2 to the service's startDate.
You might consider making Expression2's formula more easily editable and dynamic. Create an input for the expression, make it a number, call it "months" and change the expression to:
new Date(new Date().getTime() - (months*30*24*60*60*1000))