Since your SEP contribution was made in March 2016, you can allocate the portion that was not a deductible contribution for 2015 as a contribution for 2016 instead. The SEP-IRA custodian only tracks and reports the year in which a contribution is made, not for which the contribution is made. It's your responsibility to track the year for which contributions are made. If your 2016 self-employment profit is sufficient, you should end up with no nondeductible contribution that would need to be reported on Form 5330.
