can I add custom start dates for tracking for each user?

The knowledge sources don't directly address setting custom start dates per individual user for tracking purposes.

However, here's what the knowledge sources do cover that may be relevant to your use case:

If you want to track when certain user properties became active (e.g., a subscription start date), you can import historical profile data using Warehouse Connectors. This requires:

This allows Mixpanel to attribute the correct property value to each user at the right point in time. [Importing Historical Profile Values]

If your goal is to measure retention starting from a specific user action (like sign-up), Mixpanel's Retention report uses the first event (e.g., "Sign Up") as each user's individual "birth" date. You can also use custom retention brackets to define flexible time windows (e.g., Days 1–7, Days 8–30) relative to that starting event. [Custom Retention Brackets]

Could you clarify what you're trying to achieve? That would help narrow down the best approach for your use case.

I have some bad quality data for the users - where an internal user may have logged into their account. I want to keep this out of the analysis