You must take an RMD in the same tax year you turn 70 1/2. Now for 2018 there are 365 days in this year. On day 183 you will turn 70. On day 366 you will turn 70 1/2. Since there's only 365 days in 2018, that means you turn 70 1/2 on Jan 1, 2019. So you should be fine waiting until 2019 if you want. However, I would *HIGHLY* suggest you confirm this with a tax professional that you can hold legally liable and financial culpable for the advice they give you. You don't have that in this public user to user forum.
