Expiration date with a time clause.

I have a contract with TELKOM which expires on the 21st of this month. Wanting to make sure it is clear that I have no intention to upgrade/renew, I made the trip to Hatfield plaza to start the process. On my arrival the assistant tells me your contract can only be canceled on the said date at 17:05, the shop will be closed at that hour, meaning I have to return some other day. The assistant did not even offer a solution.