panky, Jenn2282 is correct.
My first response posted yesterday didn't show up but what I had suggested is that you have the option of purchasing the initial 30-days of service directly from Straight Talk at the time of activation with either a credit or debit card.
By doing it in this manner, it eliminates the hassle of transferring a code which you will have to do if you purchase the $45 service card from Walmart. It also should eliminate your concern about when service starts as you know your are paying for it on the day of activation and also eliminates the added stress of ordering a service card from Walmart for the initial 30-days of service.
Additionally, as I caution everybody else, purchase only the initial 30-days of service and no more as if it turns out that Straight Talk service is not for you, you will only be out the $45 as any service paid for in advance, as required by prepaid providers such as Straight Talk, is non-refundable (in other words, do not buy the 90, 180, or 365 day service cards initially until you determine that Straight Talk is the provider you want to use).
the 30 days does not start till you add the time to the phone NOT at the time you buy it...