This could be in the background or displayed on left side of calendar of the time entry screen but you need to add the the US ISO version of Work Week numbering (ISO-8601), There are also other schema's for determining the work week number and be part of the time entry setup to select work week or not and if selected which week numbering method. This data needs to be available for invoices, reports and time entries exports. See below for more info.
How often would you use this request?
Every week I bill time and every time I invoice.