I am assuming you are referring to NOW Broadband with the saver option 12 month contract?
When you signed up to NOW Broadband or renewed for a further 12 months did you receive an email from NOW where somewhere on the email it says Product type for example Super Fibre with a "minimum start date" quoted plus states minimum terms of 12 months" ?
If so the contract run out date would be 12 calender months from the minimum start date xx/xx/xxxx
If you can't find it under your email address old read folders, then see if it's still there in your NOW online account under Broadband & Calls > Messages.
If this is your first year with NOW then usually your live activation date is the start of the 12 month contract period (go to Broadband & Calls > Orders & Appointments to see if the live activation date on your account is still showing, or if you still have the Router box packing there should be a sticker somewhere on the box with your live activation date).
Otherwise get in touch with NOW Broadband.
Also, when you get closer to your 12 month anniversary, NOW will send you an email reminder with options and dates etc.
Here's an email address for NOW (see screenshot below) if you can't find the information you are after.
Look at your bank statements to see when you started paying Now for broadband.