This is what I came up with, but I suspect it's effectively just a combination of original code and Lynn's....
CREATE TABLE #Devices (
    DeviceUid uniqueidentifier NOT NULL PRIMARY KEY...

If once you do that LEFT JOIN, you get more records than you started with, then you must have multiple rows in the LEFT JOINed table that match any given...

Basically, matching only on the last 4 isn't going to give you a correct result, as two people can easily have the last 4 of their SSN be the same. ...

Question is, are the values in the database "per unit time", or in some way cumulative?   It's important to understand exactly what one row in the database table actually represents. ...

And here's the rest of the code that handles the last Sunday in both March and October, with a test rig to produce results for the entire period from date...