On the Backdoor Roth IRA, Does the Convert Need to be the Same Year?
The conversion doesn't need to occur in the same year, but not doing so may defeat the purpose of avoiding taxes.
Example: If you make a $5,500 non-deductible contribution to a Traditional IRA and then convert it to a Roth IRA a year later when it's grown to $6,500, you'll have to pay taxes on the $1,000 of growth. To avoid that, most such conversions take place shortly after the initial contribution is made (although the "step transaction doctrine" should be considered). There may still be some taxable income in the conversion, but it can often be kept to a minimum.
Remember as well the pro-rata rule. If you already have other Traditional IRAs with either deductible contributions or growth (in other words, pre-tax), you won't be able to just convert the latest non-deductible contribution. You'll have to pro-rate the conversion.
Example: If you make a $5,500 non-deductible contribution to a Traditional IRA and then convert that, but you already have $49,500 in (pre-tax) Traditional IRA account, your non-deductible contribution will be limited to 10% of your total Traditional IRA balances of $55,000. So only $550 of the $5,500 conversion would be tax-free.
If this is your case but you have a 401(k) or other retirement plan through work, you may be able to transfer your pre-tax IRA balances to your 401(k), if the plan will accept such incoming transfers. Then you could make your non-deductible contribution to a Traditional IRA and it will comprise 100% of your IRA assets for "back door" conversion to a Roth IRA. (On the other hand, there may be good reasons for keeping any pre-tax IRAs separate from retirement plans at work.)
There are several issues and potential trade-offs to consider. Before proceeding, I'd encourage you to consult an advisor to review this "back door Roth conversion" strategy in the context of your broader financial picture.
Hope this helps. All the best!