The closest thing I've found to IRS guidance is in the IRS discussion of the Tax Reform Act of 1986 where it states in the discussion of the newly allowed ability to make nondeductible traditional IRA contributions:
C2: What is the rule for removing excess contributions?
A: Prior to the Act, section 408(d)(4)of the Code permitted individuals who had made excess contributions to an IRA to withdraw such excess IRA contributions with earnings by April 15 of the year following the year for which the contribution was made. By withdrawing the excess in such manner, the individual was able to avoid the excise tax under section 4973 of the Code applicable to excess IRA contributions. Under the Act, amounts contributed to an IRA for an individual's taxable year (both deductible and nondeductible contributions) may be treated as excess contributions and withdrawn by the individual by April 15 of the following year. Generally, if an individual is not yet 59-1/2 at the time of the withdrawal, upon withdrawing such amounts the individual will be required to pay the early withdrawal tax under section 72(t) on the earnings (if any) for the year for which the contribution was made.
This refers to removing the contribution by April 15 of the year following the year for which the contribution was made.
