You're not the first to ask this recently and I don't understand why anyone would assume it worked like that. When you redeem the code your 30 days starts right away. If you don't want to start your 30 days then don't redeem it.
Also, incase you didn't know, you need a credit card upon redeeming your code, which you can remove right after you redeem it. If you decide to not remove your credit card information, Plus has an auto-renew feature set to on so you'll be charged after the 30 days is up. If you don't want to be charged after the 30 days you simply turn auto-renew off by going to the Playstation Network tab on the XMB: Account Management > Transaction Management > Services List > Playstation Plus > Turn Off Automatic Renewal