OTL Time Entry page modification-projects layout

We are using the seeded projects layout, to enter time via self service. The 'Period' drop down list, defaults to current week. So for e.g. on monday's the default week is the current week, but users need to enter hours for last week, and if they forget to change the week in the drop down list, the hours go in the wrong week, which then creates manual work at the back end.
We want to modify the 'Period' lov to display a blank row (basically no period) which will force the users to select a week.

What package/procedure should we be modifying for this change. Need some help to point us in the right direction.