The knowledge sources do not contain enough information to answer this precisely. The documentation does state that the Experiment Start Date defaults to the "date of the first user exposed to the experiment" [Experiments Setup], which in your case would be May.
However, there is one important constraint to be aware of:
You cannot analyze experiments prior to your experimentation purchase date. [Experimentation Pricing FAQ]
So the answer to your timing question depends on a few unknowns:
If the start date anchors to May (first exposure), the 60-day duration would end around end of July.
If the start date is treated as June (when you created the report), it would end around end of August.
The docs don't explicitly clarify how the 60-day duration interacts with a backdated start date when the report is created later. I'd recommend reaching out to Mixpanel Support or your account team to confirm the exact behavior before relying on either assumption.
