How should I structure my time based index?
If a single column is used by a query for filtering, it can use an index if the that column is the left-most column in the index.

So assuming the TimeSheetDate is the left-most column in your index, you don't need to create another index for it.

Whether or not you should create another index in a reversed order is dependent on the selectivity of the UserCustomerKey column and whether queries filter using it.