If you want users to enter expenses for a specific period of time then you need to set the "Expense Incur Start Date" and "Expense Incur End Date" in the project structure.
 
When users try to enter an incur date which is not between the "Expense Incur Start Date"  and "Expense Incur End Date" specified in the project structure then the following error will be thrown:
 
Error: The date of the expense must be within the expense date range defined for the project.
 
"Expense Incur Start Date"  and "Expense Incur End Date" can be setup in the project structure, this feature will work only for expenses which require project name to be selected.