It sadden me to know that you are asking this kind information. But to answer your concern, yes, you are correct. That is why highly suggest to set it in the end of the billing cycle to avoid any prorated charges. If you don't mind me asking may I know the reason why you would like to know this information? 

  

Customers who cancel mid-cycle, whether they have or haven't received their bill yet, should pay their bill as usual. They will receive an adjusted final bill, reflecting any adjustments or bill credits owed. 

  

https://c2.t-mobile.com/accounts/final-bill-expectations-and-disclosures