Add a week counter display format

I would like a display format option that shows in week numbers, but displays like a counter from a particular date I have identified. The later part isn't an issue, since I can identify this in the arguments, but the functionality of displaying a week number is what I need.

In essence, this is exactly like the "time ago" display format (e.g. "2 weeks, 1 day ago"). Instead, I would like to have it show as "Week 1", "Week 2", etc. based on the starting date I have identified.