Does my onward flight need to be within 30 days if I plan to extend my tourist stay in Thailand?

Hi, I have a quick question I'm hoping someone could answer for me. I'm going to be flying to Chiang Mai next month and plan on getting a 30 day tourist visa on arrival..While I'm there i plan on getting a 30 day extension for a total of 60 days. Does the onward flight i show on arrival need to be within the initial 30 days or will they except an onward flight 60 days after my arrival since i plan on getting a 30 day extension? Thank You