Calculated Calendar field - cannot change date?

I have two list columns, start date and end date.   Start date is a standard date picker, but end date is a calculated field which adds 3 months to the start date.  Is there any way I can set the list up to "offer" a date 3 months later by default but then allow someone to specify their own date?   for example:

Start Date - 09/08/2012 - Calculated date is offered as 09/11/2012 but can be edited to suit?

Regards

Mark