Basic modelling question

We have two tables
1. Streaming usage fact (holds the subscription info we have for customers, i.e. they pay us $20 a month for the usage of our product)
2. Customer dimension (hold username etc)

A colleague and my self are having a discussion on where the start date of a customer (date their account was created).
I believe it should be added to the Customer dimension as it is something that describes the customer ,is set in stone and is non additive. my colleague thinks it should go into our subscription fact table as the first entry in the subscription.

Where should this nugget of info go?

It should be in DIM table. When you store this record key in fact table you can get customer information along with their subscription date.

It should a FK in the fact to the date dimension.