Interest on tbills is paid upfront!!
Lets try and make this simple!
Imagine your father gave you 100million to invest with a compulsory return of 5million on top after one year... Anything over and above is for you to keep!

You have a choice to choose between tbills and fixed deposit at 10% each... (assuming no withholding tax)
1. You have an option of depositing the 100million in a bank to receive 10million in interest at the end of the year... You will pay your father 105million and keep 5million for your own pleasure! But you will have to wait one year to enjoy this money...
2. You have an option of buying tbills meaning you will pay 90.9million and receive 100million at the end of the year! But you will be remaining with 9.1million (100-90.9)... You then pay your father his 5million interest TODAY and you enjoy your 4.1million interest TODAY and give him the full 100million at the end of the year!!!
doesn't this logic prove that tbill interest is upfront??
Some will wonder why one would choose 4.1million today instead of 5million at the end of the year! I say to those that thanx to the TIME VALUE OF MONEY the two amounts are equal!
Mark 12:29
Deuteronomy 4:16