Offset for Expense Planned Date

When using Expense objects at the task level, the Expense Planned Date is based on the task's Planned Completion Date.  Is there a way to add in an offset from the task's planned completion date to the planned date for the expense?

In our projects the Task respresents the activity occuring; however, we do not realize the expense until ~1 Month later. This frequently resulsts in project expenses falling outside of the monwth/quarter, where the expense was originally planned.  Adding an offset between Task Planned Completion and Expense Planned date, is desireable to provide more definition on when project activities occur and when the expenses are realized. 

I would like to be able to make the Expense Planned Date = Task Planned Completion Date + 20days.  Is there  a way to make this adjustment at the project, or even the Task/Expense level?