Hello. I haven't had any luck getting help on this question

I have products and I have a daily average for each month by product. HOWEVER, when a product is in it's first month, sometimes it starts on the 15th of the month as an example, but the formula I'm using counts all days in that month so the average is lower than it should be. How can I accommodate for these types of products (they are the ones with 0's in a lot of months)?

This is the average formula I'm currently using:

AVG({ FIXED [Category], [Date]: SUM([Sales Volume])})