I dont know about the earlier process. But now you get the assignment in the below mentioned site:
https://ibt1.prometric.com/login.asp
In the login, You have an option to get the receipt. I think you can find the downloaded date in there.
If you have a similar site from where you downloaded the assignment, I suggest you to check for that option.
Else if you have paid with some credit card, You can check the card statement:-)
Regards,
Venkat