Calculate the portion of a planned number during a period, based on another time period

[BryGuy81]

I have "things" which should deliver a planned amount evenly over a start and end date. I am trying to use a weekly report date (which will change week to week) to calculate how many of those "things" should have been delivered over that time period.

I figured out how to make it work if the weekly reporting period is completely outside/inside the time period, but cannot if only a few days fall into the star and end date.

How can I calculate the number of days one date range overlaps another?